Surface Modification Of Ni-rich Layered Oxide Cathode For Lithium-ion Batteries

Ni-rich ternary layered oxides,such as Li Ni_0.8Co_0.1Mn_0.1O₂(NCM811)and Li Ni_0.8Co_0.15Al_0.05O₂(NCA),have been regarded as the most promising cathode for lithium-ion power batteries,due to their high specific capacity and low cost.However,the nickel-rich cathodes usually undergo surface degradation at highly delithiated states because of the parasitic side reactions of high valence Ni⁴⁺with electrolyte,resulting in the transition metal dissolution,rock-salt phase formation,morphological and structural destruction of the active particles,and eventually a fast capacity decay of the electrode.To stabilize the interfacial structure of nickel-rich cathodes,various strategies have been proposed to depress the parasitic side reactions by building robust cathode-electrolyte interphase(CEI),elemental doping of bulk lattice,tailoring the morphology and structure of the active particles,optimizing the composition of the electrolyte.Among these methods,building a stable CEI is a direct way to shield Ni-rich material surfaces from electrolyte corrosion and inhibit surface parasitic reactions.This master thesis presents the study of surface modifications for Ni-rich NCM811 cathode.The main contents and results are as followed.1.Modification of Ni-rich cathode with conductive polymer.Polytriphenylamine(PTPAn)was chosen as the surface modification layer of NCM811 cathode due to its good electrical conductivity and stable electrochemical activity.The modification amount of PTPAn on the surface of NCM811 cathode was optimized and also,the influence of PTPAn modification on the structure and electrochemical performance of cathode were investigated.The experimental results demonstrate that PTPAn modified layer with a mass fraction of 1 wt%can effectively improve the cycle stability of NCM811 cathode.The capacity retention of the modified cathode after 200 cycles is78.7%,which is 14.0%higher than that of the pristine cathode.The surface modification of PTPAn does not change the crystalline structure and charge/discharge processes of NCM811 cathode,but it can act as a physical barrier to isolate the direct contact of cathode surface with bulk electrolyte,thus depressing the surface side reactions and slowing down the voltage and capacity decay.2.Modification of Ni-rich cathode with ionic conducting material.To seek for a more effective surface protection,we propose a strategy to form an artificial Li⁺-conducting cathode-electrolyte interphase(ALCEI)on the NCM811 cathode surface by a purposely designed nucleophilic reaction of polysulfides with vinylene carbonate(VC).The feasibility of this strategy was verified by first discharging the sulfur contained NCM811 cathode in a VC/ether co-solvent electrolyte to in-situ form an ALCEI layer,and then reassembling and cycling the ALCEI-modified cathode in carbonate electrolyte.On this basis,the effectiveness of surface pre-modification of active particle and electrode with ALCEI layer was investigated from the consideration of practical application requirements.The experimental results show that the reaction product of the nucleophilic reaction of polysulfides with VC is a mixture of polycarbonates,lithium alkyl carbonates,lithium alkyl oxides,and Li₂CO₃,which has a high room temperature ionic conductivity of 0.16 m S cm^-1and wide electrochemical window over 4.3 V.All the ALCEI layers formed on cathode surface in three different ways can significantly improve the cycle stability and rate performance of NCM811 cathode.The in-situ electrochemically formed ALCEI layer raises the capacity retention of NCM811 cathode from 57.4%to 79.7%after 500cycles and simultaneously,increases its discharge capacity from 108.4 m Ah g^-1to133.9 m Ah g^-1at 10C.The pre-modified ALCEI layer on the active particle surface improves the capacity retention of NCM811 cathode from 78.9%to 91.6%after 100cycle and increases its discharge capacity at 10C to 125.9 m Ah g^-1.The pre-modified ALCEI layer on the cathode surface elevates the capacity retention of the NCM811cathode after 100 cycles to 94.5%.Apparently,the improved performances of NCM811 cathode arise from the dense structure and high ionic conductivity of ALCEI modified layer.This work provides a novel and facile approach for surface modification of Ni-rich cathodes.