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High-energy cathode material for long-life and safe lithium batteries

Yang-Kook Sun 1*?,Seung-Taek Myung 2*,Byung-Chun Park 1,Jai Prakash 3,Ilias Belharouak 4and Khalil Amine 4?

Layered lithium nickel-rich oxides,Li [Ni 1?x M x ]O 2(M =metal),have attracted signi?cant interest as the cathode material for rechargeable lithium batteries owing to their high capacity,excellent rate capability and low cost 1–7.However,their low thermal-abuse tolerance and poor cycle life,especially at elevated temperature,prohibit their use in practical batteries 4–6.Here,we report on a concentration-gradient cathode material for rechargeable lithium batteries based on a layered lithium nickel cobalt manganese oxide.In this material,each particle has a central bulk that is rich in Ni and a Mn-rich outer layer with decreasing Ni concentration and increasing Mn and Co concentrations as the surface is approached.The former provides high capacity,whereas the latter improves the thermal stability.A half cell using our concentration-gradient cathode material achieved a high capacity of 209mA h g ?1and retained 96%of this capacity after 50charge–discharge cycles under an aggressive test pro?le (55?C between 3.0and 4.4V).Our concentration-gradient material also showed superior performance in thermal-abuse tests compared with the bulk composition Li [Ni 0.8Co 0.1Mn 0.1]O 2used as reference.These results suggest that our cathode material could enable production of batteries that meet the demanding performance and safety requirements of plug-in hybrid electric vehicles.

Recent severe fluctuations in crude-oil prices and global environmental concerns have accelerated efforts to develop lithium-ion batteries for plug-in hybrid electric vehicles (P-HEVs).One of the principal cathode materials for such lithium batteries,LiNi 0.80Co 0.15Al 0.05O 2,has been investigated intensely in the past ten years 1.However,Li [Ni 0.8Co 0.15Al 0.05]O 2shows poor thermal characteristics because of the oxygen release from the highly delithiated state (for example,Li 0.35–0.55[Ni 0.8Co 0.15Al 0.05]O 2),which oxidizes the electrolyte and leads to a severe thermal runaway of the cell 2–4.Furthermore,the high concentration of unstable Ni 4+,when charging this material,is reduced to a divalent and insulating NiO phase at the cathode surface,resulting in high interfacial cell impedance and poor cell electrochemical performance 4–6.Even though the Ni-rich material,Li [Ni 0.8Co 0.1Mn 0.1]O 2,has poor cycle life and safety issues,its high capacity of approximately 200mA h (g-oxide)?1remains attractive 7for high-energy batteries in the 40mile electric drive P-HEVs.For this application,5,000charge-depleting cycles and 15years of calendar life as well as excellent safety are of extreme importance 8.These challenging requirements make it difficult for conventional cathode materials to be adopted in P-HEVs.

1Center for Information and Communication Material,Department of Chemical Engineering,Hanyang University,Seoul 133-791,South Korea,2Department

of Chemical Engineering,Iwate University,4-3-5Ueda,Morioka,Iwate 020-8551,Japan,3Department of Chemical and Environmental Engineering,Illinois Institute of T echnology,10West 33rd Street,Chicago 60616,USA,4Electrochemical T echnology Program,Chemical Sciences and Engineering Division,Argonne National Laboratory,9700South Cass Avenue,Argonne,Illinois 60439,USA.*These authors contributed equally to this work.?e-mail:yksun@hanyang.ac.kr;

amine@https://www.sodocs.net/doc/599802089.html,.

Interface

Bulk

Li(Ni 0.8 Co 0.1 Mn 0.1)O 2

(high capacity)

Surface

(high thermal stability)

Concentration-gradient

outer layer

Li(Ni 0.8?x Co 0.1+y Mn 0.1+z )O 2

0 ≤ x ≤ 0.340 ≤ y ≤ 0.130 ≤ z ≤ 0.21

Li(Ni 0.46 Co 0.23 Mn 0.31)O 2Figure 1|Schematic diagram of positive-electrode particle with Ni-rich core surrounded by concentration-gradient outer layer.A scanning electron micrograph of a typical particle is shown in Fig.2c.

Recently,we reported a core–shell-structured Li [(Ni 0.8Co 0.1Mn 0.1)0.8(Ni 0.5Mn 0.5)0.2]O 2(ref.9)material designed to improve the cycle life and safety of lithium batteries.The core material is Li [Ni 0.8Co 0.1Mn 0.1]O 2,which shows high capacity,whereas the shell consists of Li [Ni 0.5Mn 0.5]O 2,which provides structural and thermal stability in highly delithiated states 10–12.After carefully reviewing our analysis results,however,we found a structural mismatch between the core and the shell;voids of tens of nanometres between the core and the shell were found in the prepared core–shell powders after cycling 13,14.For example,a Ni-rich compound (core material)was believed to undergo a volume change of approximately 9–10%(ref.15),whereas the shell volume change was only 2–3%during de-intercalation of Li +ions 16.The different degrees of shrinkage within the same particle may lead to gradual separation of the core and shell,preventing the realization of high capacity,because the core part loses the pathway for the Li +ions and the electron transfer provided by the shell.This discontinuity results in a drastic decline of battery performance.

Here,we report on a novel high-capacity and safe cathode material with an average composition of Li [Ni 0.68Co 0.18Mn 0.18]O 2,in which each particle consists of bulk material surrounded by a concentration-gradient outer layer.As illustrated in Fig.1,the bulk is a nickel-rich layered oxide (Li [Ni 0.8Co 0.1Mn 0.1]O 2)to satisfy the high energy and power requirement for the P-HEVs.In the outer layer,the reactive nickel ions are gradually replaced with

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Outer layer

Distance (μm)

I n t e n s i t y (c o u n t )

c Hydroxide

Oxide

Figure 2|Scanning electron microscopy (SEM)and electron-probe X-ray micro-analysis (EPMA)results.a ,b SEM photograph (a )and EPMA line scan (b )of precursor hydroxide.c ,d SEM photograph (c )and EPMA line scan (d )of the ?nal lithiated oxide Li [Ni 0.64Co 0.18Mn 0.18]O 2.In both cases,the gradual concentration changes of Ni,Mn,and Co in the interlayer are clearly evident.The Ni concentration decreases and the Co and Mn concentrations increase towards the surface.

manganese ions to provide outstanding cycle life and safety.The resulting surface composition is Li [Ni 0.46Co 0.23Mn 0.31]O 2,which is much more stable in contact with the electrolyte than is the bulk composition.

We prepared cathode materials composed of these particles by a newly developed co-precipitation method involving the precipitation of hydroxide particles from solutions of varying Ni:Mn:Co ratios (see the Methods section and Supplementary Fig.S1).Spherical Li [Ni 0.8Co 0.1Mn 0.1]O 2particles were also syn-thesized by the co-precipitation process for comparison.The total average chemical composition of the concentration-gradient par-ticle was determined by atomic absorption spectroscopy (AAS)to be Li [Ni 0.64Co 0.18Mn 0.18]O 2.The lattice parameters for the com-pound calculated by a least-square analysis of X-ray diffraction data are a =2.871(2)?and c =14.247(3)?,with the values slightly lower than those of Li [Ni 0.8Co 0.1Mn 0.1]O 2(a =2.879(2)?and c =14.262(3)?).

To determine the local composition in the bulk and at the surface,and to track the compositional change within the particles of the synthesized material,we performed electron-probe X-ray microanalysis (EPMA)and scanning electron microscopy (SEM)on both the precursor hydroxide and the final lithiated oxide with a concentration gradient,as shown in Fig.2.The diameter of the spherical hydroxide particle is about 14μm (Fig.2a);its Ni-rich central bulk occupies about 10μm.As can be deduced from Fig.2b,the concentrations of the transition-metal elements (Ni,Co and Mn)for the hydroxide remained almost constant,[Ni 0.8Co 0.1Mn 0.1](OH)2,from the centre to 5μm towards the interface;after this point,the Ni concentration decreased abruptly from 80to 40%towards the surface of the particle,whereas the Co and Mn concentrations increased from 10%each to approximately 30%,resulting in a surface composition of [Ni 0.40Co 0.29Mn 0.31](OH)2

Binding energy (eV)

2.1 μm sputtered μm sputtered 1.9 μm sputtered 1.7 μm sputtered 1.5 μm sputtered 1.2 μm sputtered μm sputtered μm sputtered μm sputtered Before sputtering

2.1 μm sputtered 2.0 μm sputtered 1.9 μm sputtered 1.7 μm sputtered 1.5 μm sputtered 1.2 μm sputtered 0.6 μm sputtered 0.9 μm sputtered 0.3 μm sputtered 2.1 μm sputtered μm sputtered 1.9 μm sputtered 1.7 μm sputtered 1.5 μm sputtered 1.2 μm sputtered μm sputtered μm sputtered μm sputtered c

Figure 3|X-ray photoelectron spectroscopic data for the

concentration-gradient Li [Ni 0.64Co 0.18Mn 0.18]O 2.a ,Ni 2p,b ,Co 2p and c ,Mn 2p.

After the lithiation at high temperature,the particle diameter was still 14μm;however,the Ni-rich bulk shrank and the outer layer expanded,as shown by Fig.2c.This change occurred because of the interdiffusion of Ni,Co and Mn during the calcination at the inter-face between the bulk and the outer layer.As can be deduced from Fig.2d,the Ni,Co and Mn concentrations remained nearly constant from the centre to 4μm towards the interface.After this point,the Ni concentration decreased gradually from 80to 56%at the bulk/outer layer interface,whereas the Co and Mn concentrations increased from 10%to approximately 22%each at the interface.Between the interface and the surface of the particle,the Ni concen-tration continued to decrease whereas the Co and Mn concentra-tions increased.Because of the interdiffusion of the metal ions,this

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Capacity (mA h g ?1)

V o l t a g e (V )

Cycling number

C a p a c i t y (m A h g ?1)

D i s c h a r g e c a p a c i t y (m A h g ?1)

160

180

200

220a

Figure 4|Charge–discharge characteristics of Li [Ni 0.8Co 0.1Mn 0.1]O 2,Li [Ni 0.46Co 0.23Mn 0.31]O 2and concentration-gradient

Li [Ni 0.64Co 0.18Mn 0.18]O 2.a ,Initial charge and discharge curves of Li [Ni 0.8Co 0.1Mn 0.1]O 2and concentration-gradient material at 55?C obtained from a 2032coin-type half cell using Li metal as the anode (current density:0.5-C rate corresponds to 95mA g ?1);b ,cycling performance of half cells based on Li [Ni 0.8Co 0.1Mn 0.1]O 2,Li [Ni 0.46Co 0.23Mn 0.31]O 2and concentration-gradient material cycled between 3.0and 4.4V at 55?C by applying a constant current of 0.5-C rate (95mA g ?1);c ,cycling performance at 1-C rate (75mA corresponds to 190mA g ?1)of laminated-type lithium-ion batteries with an Al-pouch full cell (75mA h)using mesocarbon microbead graphite as the anode and either Li [Ni 0.8Co 0.1Mn 0.1]O 2or concentration-gradient material as the cathode (upper cut-off voltage of 4.2V).

concentration change is less steep than in the case of the precursor hydroxide (compare Fig.2b,d).The composition at the surface of the particles seemed to be Li [Ni 0.46Co 0.23Mn 0.31]O 2according to the EPMA.As per our design,the intentionally induced Ni,Co and Mn concentration difference in the hydroxide precursor resulted in a final lithiated oxide with a concentration gradient that started at the interface and continued towards the surface of the particle.

X-ray photoelectron spectroscopy measurements were made to investigate the oxidation state of each transition-metal element for the concentration-gradient cathode particles.As shown in Fig.3a,the oxidation state of Ni near the surface is slightly higher than 2+.The observed binding energies for Co and Mn coincide well with those for Co 3+and Mn 4+,respectively,in Fig.3b,c.These results are in agreement with our previous X-ray absorption spectroscopy results for Li [Ni 0.4Co 0.3Mn 0.3]O 2(ref.17);the average oxidation states of Co and Mn for Li [Ni 0.4Co 0.3Mn 0.3]O 2were 3+and 4+,respectively,and that of Ni was slightly higher than 2+.Further sputtering of the surfaces of our material did not change the binding energies for Ni,Co and Mn,as shown in Fig.3.Even though the concentration-gradient particles were etched by up to 2.1μm in depth,which corresponds to the interface region,there were no apparent changes in the binding energies for Ni,Co and Mn.The composition in the bulk of the particle was Li [Ni 0.8Co 0.1Mn 0.1]O 2,and the oxidation state of each Ni,Co and Mn ion,thus,would be preferentially trivalent 17,18.However,some of the manganese near the interface may be tetravalent.Therefore,in the final lithiated oxide material,both the concentration and the oxidation state of Ni,Co and Mn change from the bulk to the surface of the particle.

We characterized the battery performance by comparison of the Li [Ni 0.8Co 0.1Mn 0.1]O 2and the concentration-gradient cathode materials.As seen in Fig.4a,the Li [Ni 0.8Co 0.1Mn 0.1]O 2material de-livered a discharge capacity of approximately 212mA h /(g oxide).A slight decrease in capacity (209mA h g ?1)was observed for the concentration-gradient material.To assess the stability of our new material,we selected an aggressive test profile where the cells were charged up to 4.4V and cycled at 55?C (Fig.4b).Cells based on both Li [Ni 0.8Co 0.1Mn 0.1]O 2and our concentration-gradient mate-rial show a high initial capacity of approximately 209mA h g ?1,which could meet the energy requirement needed for P-HEVs.However,the cell based on the bulk Li [Ni 0.8Co 0.1Mn 0.1]O 2com-position retained only 67%of its initial capacity after 50cycles,whereas our material showed excellent capacity retention of 96%during the same cycling period,which is similar to the cell based on the surface composition only,Li [Ni 0.46Co 0.23Mn 0.31]O 2.This result clearly indicates that our cathode material can provide

high

T emperature (°C)

H e a t f l o w (W g ?1)

Figure 5|Differential scanning calorimetry traces showing heat ?ow from the reaction of the electrolyte with Li 1?δ[Ni 0.8Co 0.1Mn 0.1]O 2,concentration-gradient material Li 1?σ[Ni 0.64Co 0.18Mn 0.18]O 2and Li 1?σ[Ni 0.46Co 0.28Mn 0.31]O 2charged to 4.3V.

capacity with long cycle and calendar life even at high temperature and high cut-off voltage.

Figure 4c shows the capacity retention of the Li [Ni 0.8Co 0.1Mn 0.1]O 2and our concentration-gradient material using an Al-pouch full cell with graphite as the anode.Although the cell based on Li [Ni 0.8Co 0.1Mn 0.1]O 2retained only 80.4%of its initial value after 500cycles,our concentration-gradient material showed much higher capacity retention of over 96.5%.The poor cycling performance of Li [Ni 0.8Co 0.1Mn 0.1]O 2could originate from a structural transformation at the particle surface because of the high reactivity of the Ni ions with the electrolyte,which,in turn,could increase the charge-transfer resistance between the cathode and the electrolyte on cycling 5,19–22.By reducing the Ni concentration and increasing the Mn concentration in the outer layer,we were able to stabilize the near-surface region of the material and thus limit its reactivity with the electrolyte.Also,the concentration gradient within the particle prevents the formation of microcracks and the segregation that can occur at the interface between the bulk and the outer layer,especially if there is a sharp variation of the composition at this point.

The thermal stability and safety of cathode material during charging are important concerns in judging the suitability of the material for use in lithium ion batteries for practical

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applications such as P-HEVs.Figure5shows the differential scanning calorimetry profiles of Li1?δ[Ni0.8Co0.1Mn0.1]O2and our concentration-gradient material charged to4.3V in the presence of1M LiPF6/ethylene carbonate–diethyl carbonate electrolyte.For Li1?δ[Ni0.8Co0.1Mn0.1]O2,the onset temperature of the exothermic reaction occurred at approximately180?C,with a peak at220?C. The reduced content of Ni in layer LiMNiO2(M=Co,Mn)gives rise to reduced heat generation,but,significantly,does not improve the thermal stability23.However,our concentration-gradient material shows higher onset temperature and reduced heat generation compared with the bulk composition Li[Ni0.8Co0.1Mn0.1]O2.We propose that the stability of this material originates from the Mn4+in the surface region.For example,the onset temperature of the exothermic reaction was270?C for the concentration-gradient Li[Ni0.64Co0.18Mn0.18]O2;thus,the reaction with the electrolyte was delayed by approximately90?C compared with Li1?δ[Ni0.8Co0.1Mn0.1]O2,owing to the high stability of the outer surface composition Li[Ni0.46Co0.23Mn0.31]O2in our concentration-gradient material,which shows an onset temperature of over305?C. The total generated heat was2,303J g?1with our material,which is31%lower than that determined for Li1?δ[Ni0.8Co0.1Mn0.1]O2 (3,346J g?1).Those results were corroborated by nail penetration test (see Supplementary Fig.S4).The cell based on Li[Ni0.8Co0.1Mn0.1]O2 shows a major thermal runaway with explosion and fire;however, the cell using the concentration-gradient Li[Ni0.64Co0.18Mn0.18]O2 shows no thermal event.

The oxidation state of Ni for Li[Ni0.8Co0.1Mn0.1]O2is3and may reach3.7or3.8in a highly delithiated state,which can result in high reactivity with the electrolyte.Meanwhile,the concentration-gradient Li[Ni0.64Co0.18Mn0.18]O2has low nickel content at the surface,and the amount of Ni4+that is formed after charging should be lower than in Li[Ni0.8Co0.1Mn0.1]O2.In addition,even though the same capacity is delivered,the resulting oxidation state for the Ni would be slightly lower owing to the two-electron reaction of Ni(Ni2++2e?→Ni4+)of the composition for the outer layer.

A combined effect such as the presence of more stable Ni2+and the low concentration of Ni at the surface contributes to the thermal stability of our material.Furthermore,the surface consists of Li[Ni0.46Co0.23Mn0.31]O2,which is rich in tetravalent Mn,which leads to stable thermal behaviour in highly delithiated states.

In conclusion,we have developed a cathode material that has a concentration-gradient structure within each particle’s outer layer.This material shows not only a very high reversible capacity of209mA h g?1based on the particle bulk composition of Li[Ni0.8Co0.1Mn0.1]O2,but also excellent cycling and safety characteristics,which are attributed to the stability of the concentration-gradient outer layer and the surface composition of Li[Ni0.46Co0.23Mn0.31]O2.This material should eventually lead to advanced lithium-ion batteries that meet the P-HEV requirements. We anticipate that this novel approach should lead to the design and development of a wide range of other safe and stable,high-capacity intercalation compounds.

Methods

Synthesis of Li[Ni0.8Co0.1Mn0.1]O2.To synthesize spherical Li[Ni0.8Co0.1Mn0.1]O2, we used NiSO4·6H2O,CoSO4·7H2O and MnSO4·5H2O(8:1:1in molar ratio)as starting materials for the co-precipitation.Details of the preparation procedures are described in a previous report22.The as-co-precipitated particles were filtered and washed with deionized water.The obtained powders were dried in a vacuum state at25?C to remove adsorbed water.Finally,a mixture of LiNO3and the produced hydroxide(Li/Ni+Co+Mn ratio=1)was calcined at750?C for20h in air.Then, it was cooled to room temperature in the furnace.

Synthesis of concentration-gradient material.To prepare the concentration-gradient cathode material,we also used NiSO4·6H2O,

CoSO4·7H2O and MnSO4·5H2O(8:1:1in molar ratio)as starting materials

for the co-precipitation of[Ni0.8Co0.1Mn0.1](OH)2.During the reaction,Ni-poor aqueous solution(Ni:Co:Mn=0.08:0.46:0.46in molar ratio)was pumped into a Ni-rich(Ni:Co:Mn=0.8:0.1:0.1in molar ratio)solution tank,after which the homogeneously mixed solution was fed into a continuously stirred tank reactor. The obtained particles were filtered,washed with deionized water and dried at25?C to remove adsorbed water in a vacuum state.Finally,a mixture of the produced hydroxide and LiNO3(Li/Ni+Co+Mn ratio=1)was calcined at780?C for20h in air.Then,it was cooled to room temperature in a furnace.

Materials characterization.The crystalline phase of the prepared powders was identified at each stage by powder X-ray diffraction(Rigaku,Rint-2000)using Cu Kαradiation.The diffraction data were obtained at2θ=10?–110?,with a step size of0.03?.The morphology of the prepared powders was also observed by scanning electron microscopy(JSM-6340F,JEOL).Line scans of the polished surfaces for the prepared concentration-gradient hydroxide and calcined lithiated powders were analysed by an electron-probe microanalyser(JXA-8100,JEOL). Chemical compositions were analysed by atomic absorption spectroscopy(Vario 6,Analyticjena).X-ray photoelectron spectroscopy(PHI5600,Perkin Elmer) measurements were made to investigate the electronic state of Ni,Co and Mn for the concentration-gradient material.Macromode(about3mm×3mm)Ar-ion etching was used to determine the concentration depth profiles of the powders. The etching rate was estimated as4.2nm min?1.

Electrochemical test.For fabrication of the positive electrodes,the prepared powders were mixed with carbon black and polyvinylidene fluoride(80:10:10)

in N-methylpyrrolidinon.The obtained slurry was coated onto Al foil and

roll-pressed.The electrodes were dried overnight at120?C in a vacuum before use. Preliminary cell tests were done with a2032coin-type cell using Li metal as the anode.The cycle-life tests were performed in a laminated-type full cell wrapped with an Al pouch(thickness,1mm;width,40mm;length,60mm;capacity,

75mA h).Mesocarbon microbead graphite(Osaka Gas)was used as the anode.The electrolyte solution was1M LiPF6in ethylene carbonate–diethyl carbonate(1:1in volume).The cell was cycled between3and4.2V at a very low rate of0.01–0.5C (1.9–95mA g?1)during the initial formation process.The cells were charged and discharged between3.0and4.2V by applying a constant1-C current(75mA corresponds to190mA g?1)at25?C.

Thermal properties For the differential scanning calorimetry experiments,

the cells were charged to4.3V versus Li and disassembled in an Ar-filled dry box.A stainless-steel sealed pan with a gold-plated copper seal(which can withstand150atm of pressure before rupturing and has a capacity of30μl) was used to collect3–5mg samples.The measurements were carried out in a Pyris1differential scanning calorimeter(Perkin Elmer)using a temperature scan rate of1?C min?1.The weight was constant in all cases,indicating no leaks during the experiments.

For the nail penetration test,Li-ion batteries(thickness,1.5mm;width,

40mm;length,60mm;capacity,120mA h)charged to4.2V were penetrated by a sharp stainless-steel nail at a constant speed of4mm s?1controlled by a motor.The cell temperature and cell voltage were monitored during the test.

Received2June2008;accepted18February2009; published online22March2009

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Acknowledgements

This work was supported by the Global Research Network Program in collaboration with the US Department of Energy’s Argonne National Laboratory.

Additional information

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High-energy cathode material for long-life and safe lithium batteries

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