The Interface Chemistry And Control Strategy Of Ni-rich Layered Cathode Materials

Lithium-ion batteries have many advantages,such as high energy density,excellent electrochemical performance and low self-discharge rate.Since their successful commercial application,they have been widely used in portable electronic devices,power tools,electric vehicles,and large-scale energy storage.The rapid development of technology and economy has made people’s needs increase day by day.The development of lithium-ion batteries with high energy density,low cost,and long service life to apply to the above equipment has become an urgent task for lithium-ion battery researchers.The layered transition metal oxide cathode material(LiNixMnyCozO2,x+y+z=1,NMC)with high average operation voltage,high specific capacity and excellent cycle performance is currently one of the most likely cathode materials for lithium-ion batteries with high specific energy(>300 Wh kg-1),low cost and long life.With the increase in the proportion of Ni element and the decrease in the proportion of Co element in NMC materials,the specific capacity and average operation voltage of NMC materials would increase,and the cost would decrease,especially for Ni-rich layered cathode materials with Ni>0.8,such as NMC811.Although the increase in Ni content brings many advantages to NMC materials,many problems caused by the increase in Ni content also severely restrict its application.These problems still need to understand the reasons and find effective ways to solve them.Among the many problems existing in NMC811,the main ones that this article focuses on are as follows:(1)Since the NMC811 cathode material is more sensitive to water,the surface structure would be very unstable when stored and processed in humid air,and the electrochemical performance would be severely deteriorated.Therefore,the production of NMC811 batteries must ensure that the environment is very dry,which increases the cost of battery.And because it is more sensitive to water,it is not possible to produce NMC811 batteries using the more environmentally friendly and low-cost water-based slurry mixing process.However,there are still many controversies about the reasons why the NMC811 cathode material is sensitive to water and the electrochemical performance of the NMC811 cathode material deteriorates due to water.Only by revealing that the NMC811 cathode material is sensitive to water and the root cause of the deterioration of its electrochemical performance after being stored and immersed in water can the stable storage and processing of the NMC811 cathode material in the air be effectively improved.(2)The Ni of NMC811 with high SOC%is+4 valence,which has strong oxidizing and catalytic activity,which leads to the easy decomposition of conventional commercial ester electrolytes with low electrochemical window,which affects the cycle life of NMC811 batteries.Especially when the charge cut-off voltage of NMC811 is increased to 4.4 V vs.Li/Li+ in pursuit of higher energy density,the deterioration of electrochemical performance becomes more serious.Therefore,research on strategies to improve the stability of the electrochemical interface of NNMC811 is an urgent task for the development of NMC811 batteries with high specific energy and long cycle life.In this article,we mainly studied the air storage stability and electrochemical interface stability of the NMC811 cathode material.Through tuning the interface of NMC811,the stable storage and processing of NMC811 in humid air and the aqueous slurry mixing of NMC811 were achieved,and the electrochemical performance of NMC811 at high voltage of 4.4 V vs.Li/Li+ was also enhanced.The main contents are summarized as follows:1.Studying the air storage stability of NMC811.When NMC811 is stored in humid air and soaked in water,the fundamental reason for the formation of reconstructed layer on the surface of NMC811 is Li/H exchange.It is revealed that the surface reconstruction layer is the root cause of the deterioration of the electrochemical performance of the failed NMC811,the abnormal first charging curve and the splitting of the diffraction peaks(003)&(101)for the first charging in the insitu XRD patterns(the so-called fictitious phase,H1→H2 phase).Through the research on the first charge-discharge curve of NMC811,a new method for qualitatively judging whether the initial NMC811 fails or not is proposed via the first charge-discharge curve.2.Tuning the interface to improve the air storage stability of NMC811.NMC811 materials stored in non-dry air would seriously damage the electrochemical performance.Therefore,the storage of NMC811 must be in a dry air(dew point≤50℃),which increases the cost of battery production.In this section,molecular selfassembly technology is used to construct a perfluoro-decyl-siloxane(PFDTMS)skin on the surface of NMC811.Since the contact angle of PFDTM@NMC811 to water reaches 162°,the surface of PFDTMS@NMC811 has super-hydrophobic properties.It could make NMC811 stable storage in humid air,compared to storage in a dry atmosphere,there is no loss of electrochemical performance,reducing the storage cost of NMC811.The decomposition products of this coating layer and the electrolyte decomposition products participate in the formation of a more stable interface,which improves the electrochemical performance of the NMC811 battery.It is proved that the molecular layer self-assembly technology could comprehensively improve the air storage instability and electrochemical interface instability of NMC811.3.Tuning the interface to realizing Ni-rich cathodes preparation by aqueous processing.Since the surface of NMC811 would form reconstruction layer under the induction of water,there is no way to prepare the NMC811 electrode using the waterbased slurry process.In this section,we use vapor deposition process to coat the nonhydrophobic amino-siloxane uniformly on the surface of NMC811,isolate the contact between NMC811 and water,maintain the surface structure of NMC811,and initially achieve the aqueous slurry of NMC811.APTMS@NMC811 has greatly improved electrochemical performance compared to the bare NMC811 prepared by aqueous slurry.This process could reduce the cost of preparing the NMC811 electrode,which is beneficial to reducing the cost of battery manufacturing.4.Tuning the electrolyte composition to achieve high specific energy battery.A stable CEI is constructed by in-situ electrochemical decomposition of lithium difluoro-oxalate borate(LiDFOB)and tri-methoxy-silyl phosphate(TMSP)to coat NMC811,which improves the electrochemical performance of the NMC811 battery.Taking LiDFOB as the electrolyte additive model and NMC811‖graphite battery as the research system,a systematic and complete method is summarized to study the mechanism of film-forming additives to improve battery performance and how to choose film-forming additives.