Control over large-volume changes of lithium battery anodes via active–inactive metal alloy embedded in porous carbon

Available online at RAPID COMMUNICATION Control over large-volume changes of lithiumbattery anodes via active–inactive metal alloyembedded in porous carbon Nasir MahmoodJinghan Sarish Rehman, Quan LiYanglong Hou aDepartment of Materials Science and Engineering, College of Engineering, Peking University,Beijing 100871, ChinabDepartment of Physics, The Chinese University of Hong Kong, Shatin, New Territory, Hong Kong, China Received 14 February 2015; received in revised form 26 May 2015; accepted 29 May 2015 Available online 6 June 2015 Lithium ion battery; Large volume changes and limited access to redox sites of high capacity anode materials are great challenges. Although, various strategies were adopted but still results are far from required values for Long cyclic life; their practical usage. Here, we have designed a unique structure to prevent surface reaction and structural disintegration meanwhile intrinsic conductivity is improved to involve all redox sites in conversion reaction. CoSn x@C–PAn hybrid was synthesized through aqueous chemical route, Co doping in tin make accessible all redox sites by faster conduction of electrons while its hard nature relaxesinternal stress, carbon shell prevents surface reaction and brings well control on solid electrolyteinterface (SEI) film by maintaining barrier between electrode surface and electrolyte and nitrogendoped porous carbon provides faster diffusion of Li+ deep in electrode make possible high massloadings and conduction highway for electrons. Furthermore, porous carbon also provides room tocompensate volume expansion and keeps electrode structure stable. Because of its unique structurehybrid shows excellent reversible capacity of 2044 mAh/g (retention 100%) with mass loading of3.8 mg/cm2 along with long cyclic life up to 1000 cycles and bears high rate capability (20 A/g).Webelieve that present study makes possible the use of high capacity materials in applications.
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Presently, lithium ion batteries (LIBs) have got tremendousattention due to their high energy densities and have beenconsidered as promising power source for future electric E-mail address: . Hou).
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vehicles (EV) Therefore, in order to achieve the carbon (N-PGC) matrix named as CoSnx@C–PAn is developed practical applications of LIBs in EVs, there are numerous via in-situ polymerization and annealing processes. This efforts on seeking for high performance anode materials that nanostructure has multiple advantages: (1) doping of elec- possess high capacity and excellent stability with long cyclic trochemically inactive Co prevents structural changes and life . Metallic tin (Sn) is considered as a potential enhances internal conductivity of Sn to involve all redox substitute for conventional graphite anode (372 mAh/g) due sites in conversion reaction. (2) The nanosized particles can to its high theoretical capacity (992 mAh/g) and thermal accommodate large volume strains, hard Co improves CRt stability . However, structural disintegration, larger and electrochemical active Sn brings about high capacity.
diffusion barrier, limited access to redox sites and loss of (3) The mixed phase provides large grain boundary densities electrical contact have long been identified as primary for enhanced interfacial Li + storage, channels for faster Li + reasons for capacity loss and poor cyclic life of Sn-based diffusion by reducing the diffusion path below 10 nm.
anodes . Although nanochemistry plays a critical role (4) Carbon shell completely encapsulates nanoparticles to accommodate volume strains by developing nanosized (NPs) and thus prevents surface reactions by controlling structures of Sn, their use is still limited by unstable solid direct contact with electrolyte and limit SEI on outer side electrolyte interface (SEI) layer on the surface and poor with controlled thickness. (5) The carbon framework pro- internal conductivity As the organic electrolytes vides faster transport highways for electrons and Li + via decompose at the working potential of o0.5 V vs. Li+/Li and pore walls and pores, respectively. (6) High surface area and forms a thin SEI layer . But the expansion and well-defined pore size distribution improves the active contraction of Sn during alloying and de-alloying causes surface area of the hybrid minimizing percentage of inactive deformation and breakage of the SEI layer, respectively material through deep transport of electrolyte and make . As a result, formation of new SEI on freshly possible high mass loadings. (7) The existence of large exposed Sn surface eventually block Li+ transport via accu- nitrogen contents and graphitic ring (make possible Li6/C6 mulation within SEI and causes poor Coulombic efficiency (CE) conversion reaction) improve the overall conductivity and of cell As a consequence, the capacity decays as SEI electrochemical activity of hybrid to bring high perfor- thickness increases while most of Sn active material remains mance. With such a rational design, CoSnx@C–PAn hybrid electrically connected . However, nanosized designing has shows excellent discharge capacity of 2044 mAh/g with improved the cyclic life with enhanced performance but extraordinary CE and CRt of 100% after 100 cycles at simple reduction of crystallite size to nanoscale has intro- 0.2 A/g with mass loading of 3.8 mg/cm2. Furthermore, duced new fundamental challenges . Large surface hybrid was tested at current density of 10 A/g for 1000 area exposed to electrolyte and higher surface energy that cycles to explore the stability for long cyclic life and rate increase side reactions, results in lower CE and causes capability at higher charge–discharge rates (20 A/g), inter- thermal runway which leads to internal short circuit due to estingly, hybrid shows excellent capacity (1256 mAh/g) with melting of separator –. Furthermore, low tap density high CE and CRt (100%) after 1000th cycle at current increases thickness of electrode at high mass loading and density of 10 A/g. It is believed that the presented design leads to low volumetric capacity by increasing Li+ transfer will be very helpful to overcome the large-volume changes pathway . Generally, poor electrical properties are in electrode materials for energy storage devices with much observed due to higher inter-particle resistance which is improved capacity values.
further dominated by volume changes during charging–dis-charging, affecting cyclic life of electrode drastically A Experimental section lot of efforts have been devoted by developing electrolyteblockage layers, creating void space via void engineering and The synthesis of Sn, Sn@C, PAn, N-PGC and characterization making their composites with elastically strong graphene and part are provided in supporting information.
inactive hard metals to sustain the structure and improve thecyclic life of Sn-based electrodes However, to the best of our knowledge, stability concerns for long cyclic life 2SnO4 nanoparticles and high performance with good CE and capacity retention(CR t) at higher mass loadings (43 mg/cm2) has not been reported yet for Sn-based materials. To achieve this, a careful (176 mg), CoCl2  6H2O (238 mg) and CTAB (400 mg) are design of electrode materials, is highly required, that main- mixed in 20 mL of water and stirred with heating unless tains faster and deep transfer of Li+, provides better pathway temperature reaches to 85 1C. Subsequently, 20 ml of 4 M for electron and ions movement, prevents surface reactions NaOH solution was added and the mixture was stirred for with electrolyte and keeps the structural integrity of elec- 60 min at 85 1C. After the completion of reaction, the trode . Furthermore, Han et al. also investigated theore- product was collected by centrifugation and washed six tically that fused graphitic C times with water and ethanol, repeatedly. Finally, the solid 6 can deliver 42000 mAh/g via accumulation of Li+ through reversible reaction Li product was dried at 70 1C for 6 h in a vacuum oven.
Thus, utilizing the above design with conductive networkcontaining fused C 6 backbone can bring higher capacity.
2SnO4–PAn hybrid Here, we present an architectured design to overcome the aforementioned problems associated with Sn-based Co2SnO4–PAn hybrid was synthesized via in-situ polymeriza- electrodes to bring high performance with longevity at tion of aniline monomers with Co2SnO4 NPs. Initially 50 mg low cost. An active–inactive metal alloy sealed in carbon of aniline monomers was stirred for 5 min in water and pH of shell and embedded in nitrogen-doped porous graphitic the solution was turned acidic using HCl. Afterward, the

Control over large-volume changes 50 mg of NPs was added and the reaction mixture was electrochemical properties of Sn. Furthermore, the lithium stirred further for 10 min. Finally, the addition of APS storage mechanism is also illustrated in as shown in (114 mg) as oxidant was done and reaction mixture was that lithium can be stored through intercalation stirred for 24 h. After the completion of polymerization chemistry both in the carbon matrix via Lix/C6 (X=1–6) addition of ammonia solution was carried out and stirred for conversion reaction and can contribute capacity up to further 3 h to turn the PAn salt (green) to PAn base (blue). At 2000 mAh/g depending on the number of lithium intercalated last, the final product was centrifuge and wash 6 times with and at Li+ storage at grain boundaries through space charge water and ethanol repeatedly. The Final product was dried layer. Further, the diffusion of Li+ becomes easier and faster at 70 1C for 6 h under vacuum.
through the grain boundaries inside the NPs as shown by thearrows which results faster insertion and conversion reaction.
Synthesis of CoSn In fact the porous carbon maintain the faster diffusion outside the NPs and grain boundaries maintain faster diffusion insidethe NPs as results higher rate capability achieved. Further, CoSnx@C–PAn hybrid was prepared by the thermal annealing existence of Co inside maintain the continues flow of electron process. Co2SnO4–PAn hybrid was annealed at 900 1C for to redox sites and vice versa that utilizes all the redox centers 1.5 h with the heating rate of 2 1C/min under reducing to bring higher performance, its hard nature prevents the atmosphere of Ar/H2. Co2SnO4 NPs were also treated under rapturing of carbon shell, as results direct contact of Sn with the same condition to obtain CoSnx for comparative study.
electrolyte is inhibited and SEI film was blocked outside thecarbon coating around NPs. Further surface protection blocked Results and discussion the undesired side reaction to prevent the formation oflithium dendrites on surface and keep the entire structure is presenting the synthesis strategy for CoSn of electrode stable.
hybrid; first Co To reveal the structure and composition of as-synthesized 2SnO4 NPs were synthesized via wet-chemistry using co-precipitation of respective salts. Then via in-situ CoSnx@C–PAn hybrid, the x-ray diffraction (XRD) analysis, x- polymerization, NPs encapsulated polyaniline (PAn, green as ray photoelectron spectroscopy (XPS), scanning transmission emerladine salt) were grown in rod shape morphology which electron microscope (STEM) and Raman spectroscopy were were further converted to emerladine base (blue) using NH carried out. a presents XRD patterns of Co (the yield of hybrid can be easily increased or decreased by Co2SnO4–PAn and CoSnx@C–PAn, from where it is observed controlling the amount of aniline and NPs, without effecting that Co2SnO4 NPs show their typical peaks well-matched with electrochemical properties of hybrid). To obtain CoSn standard card JCPDS no. 29-0514. An additional peak exist at hybrid, the annealing process of Co 61.41 corresponds to the (220) plane of CoO (JCPDS no.48- 2SnO4–PAn hybrid was performed at 900 1C for 90 min at a heating rate of 2 1C/min 1719). Interestingly, broad peaks were observed after PAn under reducing atmosphere, which significantly reduce grown on NPs due to its amorphous structure, as perceived from XRD analysis of pure PAn shown in which 2SnO4 NPs to CoSnx alloy and build an uniform carbon shell on the surface of CoSn confirms complete encapsulation of NPs without aggrega- NPs. Furthermore, N-PGC was obtained from PAn via annealing, has high surface area and tion. However, after annealing treatment, regular structure well-defined micropores, additionally annealing effectively of PAn was changed to porous graphitic carbon and well- removed existing oxygenated groups that cause thick SEI layer defined peaks of CoSnx were observed corresponding to It is perceived that the nanoporous carbons obtained three phases, CoSn2 (JCPDS no. 25-0256), CoSn (JCPDS no.
from the polymer via pyrolysis will form interconnected 02-0559) and CoSn3 (JCPDS no. 48-1813). Phase segregation nanochannels and electronically conductive walls for both of heterogeneous CoSn crystallites with multiple phases transport, respectively. However, nanocrystals of observed here is due to higher temperature synthesis Sn and Sn@C were also synthesized to explore the effect of It is worth noting that mixed phase has multiple active–inactive metal alloy and porous carbon substrate on the advantages over pure phase likewise, high grain boundary Schematic illustration of synthesis method for CoSnx@C–PAn via in-situ polymerization and annealing processes; further presenting the mechanism of lithium storage in the hybrid.
N. Mahmood et al.
(a) XRD patterns of Co2SnO4, Co2SnO4–PAn and CoSnx@C–PAn hybrids. (b) HAADF-STEM line profile analysis of CoSnx@C–PAn hybrid. XPS spectrums of (c) CoSnx@C–PAn hybrid and (d) PAn. (e) BET nitrogen adsorption–desorption isotherm of CoSnx@C–PAn.
(f) Pore size distribution measured using BJH adsorption.
densities enhance the interfacial Li+ storage through space well-defined reflection peaks exactly matched with layer charge and thus offer large capacity and provide JCPDS no. 04-0673, delineating pure phase of Sn NPs channels for faster transport of Li+ in the NPs ,XRD b and S5 show the elemental line profile and STEM pattern of CoSnx@C–PAn also shows two diffraction peaks at image of a CoSnx@C–PAn, respectively, line scan for bime- 26.61 and 51.71, attributed to (110) and (211) reflections of tallic alloy was obtained with energy dispersive spectro- SnO2, respectively (JCPDS no. 41-1445).
scopy (EDS) in high-angle annular dark field STEM (HAADF- Raman spectrum of CoSnx@C–PAn shows typical D- and G- STEM). The elemental distribution line for Sn (red) and Co bands with ID/IG ratio 1.09 which confirms graphitic nature (black) confirm that the alloy has a consistent chemical of carbon matrix obtained from PAn ), a little composition throughout the entire NP with higher concen- higher intensity of D-band is attributed to the existence of tration of Sn. Therefore, due to homogenous distribution NPs . Furthermore, XRD pattern of PAn after annealing cobalt can easily transfer electron from carbon shell to all treatment verified the graphitic nature of extracted carbon internal redox sites to carry on the conversion reaction and as strong broad peak near 261 ) The vice versa. Furthermore, faster transfer of electrons to existence of C =C, C–N, C–H and NH2 was further confirmed redox sites and accessibility of all redox sites through Co by Fourier transform infrared (FTIR) spectroscopy, which doping improved the CRt of the electrode even after long emphasizes the graphitic nature of carbon with nitrogen cyclic life. It is also expected that any strain generated by doping . XRD pattern of Sn is shown in Sn volume change during lithiation and de-lithiation should

Control over large-volume changes TEM images of (a) Co2SnO4–PAn hybrid and (b) CoSnx@C–PAn hybrid. (c) HRTEM image of the CoSnx@C–PAn hybrid at position 1 (the inset is enlarge image to show uniform carbon shell on NPs and position of HRTEM analysis), (d) HRTEM image of the CoSnx@C–PAn hybrid at position 2 and (e) HRTEM image of the CoSnx@C–PAn hybrid at position 3. (f) TGA/DSC curves of CoSnx@C–PAn hybrid.
be evenly distributed and compensated by hard counterpart carbon, which is crucial for limiting the thickness of SEI (Co). To further confirm the composition of CoSnx@C–PAn layer on the outer surface of NPs via preventing the direct hybrid, XPS studies were carried out for both hybrid and PAn contact of NPs and electrolyte. Surface area and porosity (and d), the peaks of core levels of Co, Sn, N and C are important factors that are beneficial in electrochemical confirms the presence of all chemical species. The concen- energy storage especially batteries to improve the Li + tration of each element calculated from XPS is 89.76%, diffusion . However, fabrication of active nanomaterials 6.08%, 2.50%, 1.56% and 0.10% for C, O, N, Sn and Co, with porous carbon substrate not only stabilizes structure of respectively, as XPS is a surface detection technique thus nanomaterials, but also provides electrical highway to lower metallic contents were found because of surface electrons via walls of pores as well as faster pathway to coverage but still the concentration of Sn is higher than ions through pores. Furthermore, nanoporosity results in Co. The presence of little higher concentration of oxygen in complete wetting of C6 fused aromatic carbon matrix to CoSnx@C–PAn than PAn suggests that homogeneous precipi- fulfill the desired conversion reaction of Li6/C6 to bring tation of single phase CoSn is difficult due to existence of maximum possible performance as discussed below .
oxidized metallic ions instead of pure metal, in accordance In present study, PAn was converted to N-PGC by annealing with XRD results. Further, high resolution Sn3d XPS spectrum process without any acid or base treatment, and high shows the existence of oxidized form of tin that is SnO2 surface area of 438.5 m2/g with well-defined pore size of (Furthermore, the existence of large C peak in 1–1.5 nm was obtained d and e). Homogeneous XPS results further indicate complete coverage of NPs by pore size distribution with high surface area provides N. Mahmood et al.
efficient mass movement and larger space for Li+ storage as advantages of higher grain boundary density and interfacial well as relaxes the strains of NPs.
Li + storage at grain boundaries (called buried interfaces) via space layer charge and brings additional capacity, the NP Co2SnO4–PAn and CoSnx@C–PAn hybrids are presented in is characterized by HRTEM at different positions as shown in . The Co2SnO4 NPs are well-dispersed and embedded the inset of c. The HRTEM analysis at position 2 in PAn rods without aggregation on surface (a).
(inset of c) shows the inter-planar distance of 0.262 Interestingly, it is found that if polymerizing reagent nm, which corresponds to (110) plane of CoSn phase that is (ammonium persulfate, APS) was added before the addition in accordance with standard card (JCPDS No. 02-0559) as of NPs, NPs only aggregate on the surface of PAn rods shown in However, d-spacing of 0.279 nm is (In fact, addition of APS start polymeriza- calculated from the HRTEM analysis of position 3 (inset of tion without NPs, but earlier addition of NPs causes poly- which corresponds to (600) plane of CoSn3 merization on the surface of NPs in presence of hydrochloric according to the standard card (JCPDS No. 48-1813) as acid (HCl) which ionizes the aniline and increase its affinity shown in Thus, HRTEM studies confirm the towards NPs, resulting complete encapsulation of NPs and proposed mechanism () of faster mass transfer well-define morphology. It is worth noting that NPs are via grain boundaries and larger Li + storage to bring higher integrated as dimer or trimer in Co2SnO4–PAn hybrid as capacity, long stable cyclic life and better rate capability.
compared to individual NPs These dimers and The content of the metallic constituents (CoSnx) are trimers fused during heat treatment, transforming into NPs calculated to be 65% using thermal gravimetric analysis with average size of 40 nm formed inside N-PGC ( (TGA) as carbon wipe-out at 488 1C through oxidation the NPs size distribution is shown in The TEM determined by differential scanning calorimetry (DSC, image of CoSnx@C–PAn reveals that NPs are evenly distrib- f). An increase in weight later near 700 1C is due uted and well embedded in N-PGC matrix. Furthermore, it is to the oxidation of metallic species. The TEM image of Sn also perceived that N-PGC matrix has multidimensional NPs (shows that NPs grew in the size of 20 nm structure due to random stacking of PAn rods to facilitate with well-defined structure confirmed through SAED ( the faster mass transportation and enhance the wettability ). A 2.2 nm thick carbon shell is also constructed on of the C6 matrix. However, the presence of circular rings pure Sn NPs (Sn@C, ) for the sake of with bright spots in selected area electron diffraction comparative electrochemical study with CoSnx@C–PAn.
(SAED) pattern of CoSnx@C–PAn clearly depicts the existence The benefits of rational design of CoSnx@C–PAn for Li+ of CoSnx alloy in hybrid () and is in accordance storage were investigated through electrochemical response with the XRD results mentioned above. It is noted that the of the obtained hybrid, by developing as anode in coin type PAn grew in three dimensional networks , cell. The redox response of hybrid was delineated by cyclic mechanism is discussed in supporting information voltammetry (CV), scanned at 0.2 mV/s and cycled between ), which provides higher surface area to hybrid while PAn 0.005 and 3.0 V vs. Li+/Li a). CV curve shows the keeps its multidimensional structure even after annealing similar redox peaks for Sn reaction with Li+ to Li4.4Sn at 1.65 treatment at high temperature (). The aim of and 0.85 V and extraction of lithium occurred at 0.27 V and surface protection of NPs was achieved through in-situ 2.1 V during cathodic and anodic sweep, respectively. The polymerization of aniline monomers on the surface of NPs peak appear near 0.5 V during cathodic scan corresponds to rather than construction of simple conduction highway for the breakdown of electrolyte to form SEI film but no peak ions and electrons. It is found that during heat treatment observed in the successive cycles that confirm high reversi- the aniline attached on NPs surface shaped a uniform shell bility and control over SEI layer thickness Furthermore, a of carbon (the inset in c) and additional PAn small peak appears at 1.35 V confirms the synergism among developed N-PGC. It is well-known that NPs size distribution the CoSnx core and N-PGC matrix It is worth noting that strongly affects the electrochemical performance and tap no peak was observed during anodic scan for Co which density of nanomaterials. To obtain narrow size distribution, demonstrates its completely inactive nature Moreover, heat treatment at different rates (2, 5 and 10 1C/min) was to confirm the high rate capability of hybrid, CV scans at carried out (). It is scrutinized that different rates were carried out, as shown in . It is as heating rate was reduced from 10 to 2 1C/min, size worth noting that similar profile of CV curves at different scan distribution became narrower and remarkably carbon shell rates were obtained, which assures high rate capability of the formed only at lower heating rate. Furthermore, longer hybrid and easy access of redox sites due to unique internal time annealing was also adopted with aim to get rid of oxide structure. In contrast to hybrid electrode, the electrode of Sn species but it result in poor size distribution which is not shows huge difference among first and successive cycles that suitable for better performance (To further confirm the formation of very thick SEI layer on the surface of confirm the structure and existence of electrolyte blockage electrode and further reduction of current intensity in layer, high resolution TEM (HRTEM) studies were carried out, successive cycles confirm the reformation of SEI from c it can be seen that the surface of NP is because raptured surface exposed fresh surface of tin that completely covered by 2.2 nm uniform carbon shell.
increases SEI film thickness via irreversible storage of Li+.
Furthermore, inter-planar distance measurements make The voltage profiles of hybrid b) exhibited typical sure the existence of both electrochemically active (Sn) electrochemical features of CoSn (here broader voltage range and inactive (Co) metals, the measured d-spacing of of 5 mV to 3 V was used for the complete exploration of 0.249 nm at position 1 (inset of corresponds to stability of electrode developed with unique composition but the (211) plane of CoSn2 (JCPDS no. 25-0256) as shown in the redox reaction is in the limits of anode cut of voltage) c. To explore the segregation of multiphase, with almost no change over 100 cycles. Further, Control over large-volume changes (a) Cyclic voltammograms of CoSnx@C–PAn at scan rate of 0.2 mV/s in the voltage range of 0.005–3 V vs. Li+/Li.
(b) Galvanostatic charge–discharge curves of CoSnx@C–PAn hybrid cycled 1st, 2nd, 50th and 100th tested at current density of 0.2 A/gin the range of 0.005–3 V vs. Li+/Li. (c) Cyclic performance, capacity retention and Coulombic efficiency of CoSnx@C–PAn for 100cycles at current density of 0.2 A/g in the voltage range of 0.005–3 V vs. Li+/Li. (d) The cyclic performance and Coulombic efficiencyof CoSnx@C–PAn at different current densities in the voltage range of 0.005–3 V vs. Li+/Li.
absence of large plateaus confirms the faster transfer of of its first lithiation process and result in capacity of lithium inside the electrode through porous carbon and grain 2037.8 mAh/g. The loss of 341.6 mAh/g capacity is because boundaries as proved by HRTEM studies c–e) of the SEI film formation that was overcame in the next cycles in contrast to the electrode that shows large irreversible with improved CE. Furthermore, the plateaus in the first storage of lithium in the first cycle and in successive cycles discharge curve well concise with CV curve and show two required large time to diffuse, resulting poor CRt ().
major changes first the lithium storage through the conversion Furthermore, it is point of ponder that faster diffusion of reaction with Sn; second through insertion in the carbon Li+ is necessary for high rate capability and the linear profile matrix as indicated by the plateau below 0.5 V that continues is indication of faster diffusion through shorter distance till the cut off voltage reaches. Further to explore the (below 10 nm) . In fact, the grain boundaries serve as practical utilization of as-developed hybrid, its electrochemi- tunnels inside the NPs for faster transfer of Li+ and Co cal performances were explored in various voltage widows provides faster electron flow to redox sites as results an (0.005 to 1.5 and 0.005 to 0.6 V) as for practical utilizations efficient redox reaction happened in a short time, thus result the upper voltage limit of anode should be restricted below in linear charge-discharge profile with improved performance 1.2–1.3 V . It is worth noting that hybrid outperformed as discussed by Gogotsi and Okubo et al. The CE is an even in the restricted voltage windows and shows the indicator of the reversibility of the electrochemical reaction discharge capacities of 1524.76 and 880.12 mAh/g after 100 at electrode . The decrease in CE usually happens due to cycles with CRt of 99% at current density of 0.2 A/g in the rupture and reformation of SEI layer, especially in later voltage range of 0.005–1.5 V and 0.005–0.6 V, respectively as cycles. After initial few cycles, CE remains nearly 100% during shown in In addition the CE values reach to 100% charging and discharging process of the hybrid electrode after initial few cycles that further emphasized the high (c and d). It should be noted that low areal mass reversibility of the hybrid in the restricted voltage windows.
loadings were frequently used to achieve the stable cycling The stable CE and high CRt demonstrated that hybrid is highly life. However, high mass loadings are needed to realize the feasible for deep diffusion of Li+ and all material is active in high performance for electrification of road market The thick electrode. To observe the effect of mass loading on the hybrid shows first discharge capacity of 2379.4 mAh/g with performance of hybrid 4 electrodes were assembled with mass loading of 3.8 g/cm2 (further the tap density of the different mass loadings (2–4 mg/cm2) and it was found that as-synthesized hybrid is 0.46 g/cm3 and this is in the desirable increasing mass loading has minimal effect on performance range for electrode material as Sn based materials show large . Furthermore, similar performance shows high volume changes) at current density of 0.2 A/g and capacity repeatability of the results and further exclude the influence retention was as high as 100% (calculated from 2nd discharge) of other factors likewise cell assembly, testing conditions etc.
after 100 cycles. While the hybrid shows 85.64% de-lithiation Stable structure at particle level is highly required to achieve N. Mahmood et al.
stable long cycling of a high mass loaded electrode, because (Rct) were measured using appropriate Randles equivalent electrode level cracking and failure can be possible even with circuit shown in The calculated Rf and Rct small changes in particle morphology which accumulate before testing is 16.39 Ω and 72.14 Ω that slightly increased to across the thickness of electrode. A successful design of the 18.53 Ω and 75.14 Ω after 1000 cycles of testing, respectively.
hybrid structure is indicated by stable and excellent perfor- Vertical spikes confirmed the capacitive behavior of hybrid but mance at high mass loading. To explore the rate capability, a little increment in ions diffusion resistance (Warburg impe- hybrid was tested at various current densities and it is found dance, W) was observed after long cyclic life. Better electron/ that hybrid keeps its reversibility with changing current ion conductivity over prolonged cycles with strong resistance densities as shown by stable CE d). The hybrid keeps control for successive charge–discharge cycles suggest extra- its reversible capacity as high as 551 mAh/g at higher current ordinary CRt, CE and high stability of hybrid. To the best of our density of 20 A/g and restores its excellent performance of knowledge, long stable life with excellent performance at such 2040.5 mAh/g with decrease in current density to 0.2 A/g high mass loadings is rarely reported for Sn-based electrodes.
d). Thus, high rate capability along with higher Synergistic effect of Co doping, C coating and N-PGC with performance proves the reliability of the developed method Sn was analyzed by comparative electrochemical studies of to control over large volume changes of anode materials.
PAn, Sn, Sn@C, Co2SnO4, Co2SnO4–PAn, CoSnx and CoSnx@C– Furthermore, hybrid was tested for 1000 cycles at 10 A/g to PAn. It is simply perceived from that after making evaluate the long cyclic life of electrode. It is found that hybrid hybrids of two structures the performance increased, which keeps a capacity of 956.4 mAh/g after 1000th cycle with high indicated that materials took successfully the advantages of CRt of 98.5% (only 0.0015% capacity loss per cycle) and stable both components for better performance due to strong CE around 100% The stability of structure after interfacial interactions. The performance of Sn@C is electrochemical test was examined through TEM and HRTEM increased after carbon coating but still a large capacity loss after washing electrode material with acid to remove SEI layer was observed in the initial cycles. The fact that increasing ). From microscopic analysis, it is evaluated that the the electronic conductivity of Sn via carbon coating can structure (measured d-spacing of 0.199 nm corresponds to (310) improve the performance through faster Li+ diffusion, but plane of CoSn2 (JCPDS no. 25-0256)) and size of NPs in hybrid lithium intercalation reaction (Li4.4Sn) may be limited by are preserved after long cycling test. Moreover, the stability electron transfer between the carbon coating and redox site concern of hybrid was examined by electrochemical impedance in the crystal It is noticeable that when the intrinsic spectroscopy (EIS) before and after 1000 cycles .
conductivity of Sn was increased through Co doping, an Similar Nyquist profile was obtained for hybrid before and after improved performance was achieved but capacity loss at 1000 cycles as both curves show semicircle in high frequency initial cycles still exist because of surface reactions. Thus, by region and a straight vertical inclined line in lower frequency.
combining Co doping and constructing shell of carbon in one The SEI layer impedance (Rf) and charge transfer resistance (a) Cyclic performance, capacity retention and Coulombic efficiency of CoSnx@C–PAn for 1000 cycles at current density of 10 A/g in the voltage range of 0.005–3 V vs. Li+/Li. (b) Nyquist plots of CoSnx@C–PAn hybrid before and after 1000 cycles of charge–discharge in the range of 100 kHz to 10 mHz at open circuit potential (the inset is Nyquist plot with equal x–y axis ratio at highresolution). (c) Comparison of discharge capacities of CoSnx@C–PAn, CoSnx, Co2SnO4–PAn, Co2SnO4, Sn@C, Sn and PAn at currentdensity of 0.2 A/g in the voltage range of 0.005–3 V vs. Li+/Li.
Control over large-volume changes performance is attained with high CRt. Further the incor- of 3.8 mg/cm2. Furthermore, hybrid shows long cyclic stability poration of hard Co also prevents the rapture of C shell for 1000 cycles of charge–discharge at current density of 10 A/g around the NPs by compensating the internal stresses as over with capacity of 1256 mAh/g after 1000th cycle and impedance expansion of NPs causes the rapturing of the C shell It is calculations confirm slight change in resistances after 1000 worth noting that carbon matrix with well-defined pore size cycles. The rate capability of the hybrid is explored by testing (1–1.5 nm) provide highway to both electrons and ions at different current densities and hybrid keeps 551 mAh/g even through pore walls and pores, respectively. These pores at 20 A/g. The approach to build active–inactive metal phase provide tunnels for deep and fast diffusion of Li+ in thick encapsulated in carbon shell and embedded in N-PGC adaptable electrode to keep the entire material active, which is further matrix established here opens up a new avenue to control facilitated by the internal grain boundaries of NPs to achieve volume changes and brings high performance of tin-based faster redox reaction by shortening the internal diffusion anode, can also be extended to other attractive anode (like path. The relative sizes of Li+ ions and pores of the electrode Silicon, Germanium, etc.) and cathode materials systems that materials are also very important in determining the poten- suffer large structural changes during conversion reactions.
tial performance. In our designed system, Li+ (0.076 nm) caneasily diffuse through the electrode materials (pore size Supporting information 1.5 nm) not only providing higher performance but alsohigher rate capability . In addition, higher capacity Supporting Information is available from the Elsevier or the is observed than the theoretical capacity of Sn, this addi- author [part of experimental section, XRD of PAn before and tional capacity comes up from the interfacial storage of Li+ after annealing, Raman and FTIR results of hybrid, XRD of Sn due the existence of mixed phase by space layer charge that NPs, STEM image of hybrid, TEM images of hybrid, Co is energetically favorable at interface of two phases as shown NPs, PAn before and after annealing, hybrid at different in , intercalation of Li+ in N-PGC through conver- temperature, high resolution spectra of Sn3d and Co2p, Sn sion reaction of Lix/C6 that contributes large capacity and and Sn@C NPs, Hybrid after testing, SAED of hybrid and Sn synergistic effect between NPs and N-PGC The NPs, HRTEM, of hybrid after testing, NPs distribution graph, existence of nitrogen in the carbon make it more electro- PAn polymerization reaction mechanism, CV of Sn NPs, chemical active that results higher performance (c) capacity performance of hybrid at different mass loading, and contribute lager capacity to hybrid performance. As, behavior of bulk and nanomaterials towards Li + storage, nitrogen is capable to change the electronic structure and Nyquist plot and equivalent circuit diagram].
density of state to improve the conductivity and capacity ofgraphitic carbon To further explore the synergism ofdifferent components, EIS studies were done and represen- tative Nyquist plots are shown in In the light ofabove results, the optimized cycling stability and rate This work was supported by the NSFC-RGC Joint Research capability of hybrid is attributed to the enhanced transport Scheme (51361165201), NSFC (51125001 and 51172005), kinetics and structural stability. The high transport kinetics Beijing Natural Science Foundation (2122022), Aerostatic comes from N-PGC conductive network that provides faster Science Foundation (2010ZF71003) and Doctoral Program of electronic highway from current collector to active material the Ministry of Education of China (20120001110078).
and among the separated NPs as well as Co improves theintrinsic transfer of electron from carbon to redox sites which Supporting information make available all the material for higher performance. Thehomogenously porous structure and anisotropic adhesion Supplementary data associated with this article can be realizes efficient mass transfer of Li+. Furthermore, the found in the online version at structure is protected via accommodating internal strains by hard Co, preventing surface reactions by carbon shell andpresence of adaptable matrix of N-PGC.
In summary, CoSnx@C–PAn hybrid has been fabricated utilizing low-cost and large scale aqueous chemical growth under industrially acceptable conditions to control large volume changes and make accessible all active sites of Sn-based anode via active–inactive metal phase embedded in N-PGC matrix.
The hybrid efficiently took the advantages of mixed phase, enhanced internal conductivity by hard Co doping, surface protection by carbon shell and high surface area (438.5 m2/g) with well-defined pore size (1–1.5 nm) to improve the transport kinetics, lithium intercalation reaction (Li6/C6) in fused C6 aromatic ring of N-PGC and structural stability. The hybrid possess high capacity of 2044 mAh/g with extraordinary CE and CRt of 100% after 100 cycles at 0.2 A/g with the mass loading

N. Mahmood et al.
Nasir Mahmood obtained his BS degree in 2009 in Chemistry from Punjab University and MS degree in 2011 in Materials and Surface Engineering from National Univer- sity of Science and Technology, Pakistan. He joined Peking University in 2011, where he is currently pursuing his Ph.D in Materials Science and Engineering under the guidance of Prof. Yanglong Hou. His research involves the synthesis of graphene/graphene-based nanomaterials and their application in energy storage and conver- sion devices.
Jinghan Zhu received her B.S. in Materials Science and Engineering from the University of Science and Technology Beijing (USTB, China) in 2011. She has been pursuing her Ph.D under the supervision of Prof. Yanglong Hou in the Department of Materials Science and Engineering at Peking University since 2011. Her research interests are the chemical synthesis of graphene based nanomaterials and their magnetic and catalytic applications.
Sarish Rehman obtained his BS degree in Chemistry from Peshawar University in 2010 and her MS degree in 2013 in Materials and Surface Engineering from the National Uni- versity of Science and Technology, Pakistan.
She joined Peking University in 2013, where she is currently pursuing her Ph.D in Materi- als Science and Engineering. Her research focuses on the synthesis and development of novel nanomaterials for the application in energy storage and conversion devices.
Quan Li received her Ph.D in Materials Science and Engineering from Northwestern University in 2001. She joined The Chinese University of Hong Kong as an assistant professor in 2002, and was promoted to full professor in 2011.
Her research focuses on functional materials, including the fabrication and assembly of nanomaterials, characterizations and mea- surements of individual nanostructures, and their applications in energy and biomedicine.

Control over large-volume changes Yanglong Hou received his Ph.D in Materials research interests include the design and chemical synthesis of Science from Harbin Institute of Technology functional nanoparticles and graphene, and their biomedical and (China) in 2000. After a short post-doctoral energy related applications.
training at Peking University, he worked atthe University of Tokyo from 2002 to 2005 asJSPS foreign special researcher and also atBrown University from 2005 to 2007 aspostdoctoral researcher. He joined PekingUniversity in 2007, and now is a Chang JiangChair Professor of Materials Science. His


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