Surface Reconstruction For High-Performance Lithium-Rich Mn-Based Cathode And Its Full-Cell Performance Analysis

The rapid development of the electric vehicle and the upgrade of mileage have raised higher requirements for the energy density of lithium-ion batteries.The cathode material is the key point to restrict the capacity and the energy density of the lithium-ion batteries.The Li-rich Mn-based cathode series Li[Lix(MnM)1-x]O2(M = Ni,Co,Fe,et al)due to its high specific capacity(>280 mAh g-1),high work voltage(>3.6 V)and the amazing volume energy density(1000 Wh L-1)have been considered as a promising candidate of cathode material for high-energy Li-ion batteries.To achieve the high discharge capacity,Li-rich Mn-based material is usually charged to the voltage higher than 4.5 V,which accompanies with irreversible structure transformation.In the subsequent cycle,the structure of the material would change from layer phase to spinel-like phase.The layered-to-spinel phase transition is also accompanied by voltage fade,poor capacity retention and rate capability.The continuous structural transformation not only produces numerous phase boundaries during electrochemical process in the bulk of the material but also produces rock salt and amorphous structures.Therefore,maintaining the structure stable is expected to be an effective way to promote the performance of Li-rich Mn-based cathode material.In this paper,the electrochemical kinetics of Li-rich Mn-based cathode material was investigated.Then,the in-situ coating Li-ion conductor and introducing antisite layer on the surface were proposed and investigated based on the kinetics characteristic.Finally,full cells with the Li-rich Mn-based materials as cathode and graphite as anode were assembled and investigated.The electrochemical kinetics in the solid phase and solid-liquid interface were analyzed by cyclic voltammetry(CV),galvanostatic intermittent titration technique(GITT)and electrochemical impedance spectroscopy(EIS).The results show that the Li+ diffusion coefficient drastically decreases to 10-17-10-18 cm2 s-1,and the charge transfer impedance increases from 179 ? to 695 ? when the charge voltage is higher than 4.5 V corresponding to the activation process of Li2MnO3.The process is accompanied with the surface densification and the Mn defects in the Li-layer.When the discharge voltage is less than 3.5 V corresponding to the reduction process of MnO2,the Li+ diffusion coefficient reduced to 10-16 cm2 s-1,and the discharge transfer impedance increases from 199.6 ? to 1517?.In addition,the structural transformation gradually penetrated into the bulk of the material,which resulted in very low lithium diffusion coefficients and the decay of specific discharge capacity and the average discharge voltage.Combined with the surface characteristics of Li-rich Mn-based cathode materials,BPO4 was proposed to encapsulate Li1.16(Ni0.25Mn0.75)0.84O2 particles and expected to form Li-doped LixBPO4+x/2 layer as a Li-ion conductor on the particle surface.In this paper,the surface state and electrochemical properties of modified material were characterized to further reveal the details of the interfacial modification.First,a thin layer with about 3-5 nm in thickness covered uniformly on the surface for the modified material and the residual Li2CO3 on the modified material was reduced by about forty percent compared with the pristine sample due to form the Li-ion conductor LixBPO4+x/2 layer.Moreover,the LixBPO4+x/2 layer provided Li-ion diffusion path at the solid-liquid interface which improved the surface structural stability.While,the 1.6 mol%modified sample delivered discharge capacities of 181.3 mAh g-1 and 138.4 mAh g-1 at 2 C and 5 C.retaining 80.2%and 61.2%of the capacity at 0.5 C.In addition,the LixBPO4+x/2 layer greatly suppressed the side reaction of electrolyte at an elevated temperature(55 0 C)and improved the high temperature cycling stability of the materials.To stabilize the Li-ion diffusion path and the reversible Li+ intercalation,an antisite-defect nanolayer(transition metal ions replacing Li+ in Li slab)was induced on the surface of Li1.16(Ni0.25Mn0.75)0.84O2 by doping with boracic polyanions.In this paper,the electrochemical properties and structural characteristics of antisite-defect material were analyzed to reveal the relationship between the antisite-defect layer and structural evolution of Li1.16(Ni0.25Mn0.75)0.84O2.First,the antisite-defect layer could suppress the structure transition from layered to spinel-like phase during cycling.The 2 mol%and 3 mol%BO33--doping samples delivered discharge capacities of 205.2 mAh g-1 and 199.8 mAh g-1 with capacity retention of 91.2%and 93.7%after 300 cycles at 0.5 C.Moreover,the antisite-defect layer inhibited the formation of amorphous structure during cycling and improved the reversible Li+ intercalation.It is clarified that the antisite-defect layer with the TM ions location in Li slab hinders the generation of the Li-ion vacancies even at a deep charging state(>4.5 V)and becomes the obstruction for the continued migration of much more TM ion to the Li slab through tetrahedral sites,which mitigate phase transformation from layer to spinel during cycling.Finally,we investigated the performance of 2032 coin cell containing Li1.16(Ni0.25Mn0.75)0.84O2-based positive electrode and graphite-based(G-360)negative electrode.The effects of capacity ratio of negative electrode to positive electrode,forming voltage and cycling cut-off were also discussed.In addition,the pouch cells with Li1.16(Ni0.25Mn0.75)0.84O2 as positive electrode and graphite-based as negative electrodes were assembled,and the charge/discharge characteristics and cycle stability were discussed.The results show that the capacity ratio of negative electrode to positive electrode determines the potential of positive and negative electrode at the cut-off of the pouch cell.A higher capacity ratio would result in the higher potential of the positive and negative electrodes at the end of pouch cell discharge,which would lead to the incomplete discharge of the positive electrode.A smaller capacity ratio would result in a negative potential for the negative electrode at the end of pouch cell charge,which contributes to Li plating.At the same time,the charge transfer resistance and the solid electrolyte interface resistance of the full cell gradually increased during cycle,which suggest that the surface structural stability and ionic transport properties are the key factors affecting the cycle performance.