| Lithium ion batteries,which own the best performances among available energy storage technologies,have undergone rapid development.Unfortunately,they are still beset by the inadequate energy density in meeting the requirements of long endurance for portable electronic devices and electric vehicles.Within lithium ion battery,the cathode make up a substantial part of cost and weight(～40%)while delivers poor capacity and structural stability,which make it the key component that determines the energy density,safety and cost of lithium ion batteries.Exploring advanced cathode materials with high specific energy density and long lifetime is in urgent need.Among numerous cathode candidates,Ni-rich Li Nix Coy Mnz O2 layered oxides are considered to be the most promising,due to its high capacity and low cost.Nevertheless,the inferior cycling stability and poor rate capability of Ni-rich cathode materials,induced by the structural instability,parasitic electrode/electrolyte interface side reaction and the significant volume changes during Li+ insertion and extraction,have to be overcome before they can compete in practical implementation.However,the current modification methods are mainly performed on the secondary particle level,such as surface coating and ion doping,and most of them usually solve only one of the special pitfalls while worsening others.In this research,in order to boost the performances of Ni-rich cathodes,based on in-depth understanding of the structure-activity relationship of Ni-rich cathodes and fine-control of coprecipitation synthesis,original works are performed and promising cathode materials with high energy density,long cycle life and low cost are obtained.Directed by crystallographic and charge-discharge kinetics of the materials,effective optimizations are carried out from the aspects of morphology,structure and composition.Besides,the synthetic process used in this study is compatible to industrial production,and the batch output reaches up to 100 kg level.Concerning the hierarchical structure of secondary particles with nickel-rich cathodes,in the first part,from the prospect of morphological modulation,we propose an advanced Ni-rich cathode material Li Ni0.8Co0.1Mn0.1O2 with radially aligned single-crystal primary particles,which is massively synthesized using a fine-controlled co-precipitation method.With the radially oriented single-crystal primary particles penetrating from surface to center,all the exposed surface is the electrochemically active {010} planes,and Li+ diffuse straightforwardly from center to surface without crossing grain boundaries,building up facilitated three-dimensional Li+ channels.Moreover,the radially oriented primary particles with consistent crystal orientation might significantly alleviate the volume-change-induced intergrain stress by coordinated expansion and contraction,which can remarkably depress the pulverization of secondary particles and promote cycling stability.In accordance with this unique structure,superior reversible capacity(203.4 m Ah g-1 at 0.1C rate),rate capability(152.7 m Ah g-1 at 5C rate)and cycling stability(95.5% capacity retention after 300 charge-discharge cycles)are obtained.In regard of the detrimental role played by grain boundaries inside the secondary particles,which largely restrict the electron and Li+ transportation as well as structural instability.In the second part,from the prospect of structural modulation,we report a primary-grain engineering strategy to directly fine tune the chemistry and local structure in grain boundaries of Li Ni0.8Co0.1Mn0.1O2 cathode material without any constituent alteration.By introduction of KMn O4 during the coprecipitation,valence of the transition metals at the grain boundaries can be tuned which lead to distortion of TMO6 octahedral by the Jahn-Teller effect of Ni3+.Comprehensive methods,including in-situ synchrotron X-ray diffraction(XRD),soft and hard X-ray absorption spectroscopy(XAS),scanning transmission electron microscopy coupled with electron energy loss spectroscopy(STEM-EELS),and pair distribution function(PDF)have been applied to probe the crystal structure and electronic structure in both long-range and short-range scales.Moreover,the first-principle calculation has been also performed,which offers deeper understanding of the structure-performance relationships regarding to the grain boundaries.It is revealed that the high concentration of Ni3+ ions in the grain boundaries induces local Jahn-Teller distortion of the octahedron layers,which stabilizes the internal structure from phase transitions,and also offers a three-dimensional path for ionic and electronic transportations.This strategy leads to both dramatically enhanced long-term cyclic stability(92.3% capacity retention after 80 cycles at 0.1C rate),outstanding reversible capacity(203.8 m Ah g-1)and rate capability(152.4 m Ah g-1 at 5C rate)revealed by multiple proofs of electrochemical tests.In light of the pressing problems of parasitic side reaction and phase transformation at the cathode surface,as well as the intense strain inside the secondary particles induced by Li+ insertion and extraction.In the third part,from the prospect of compositional optimization,we propose a unique progressive concentration gradient Ni-rich cathode material,which possesses a continuously accelerated decrease of Ni(corresponding to increase of Co and Mn)from Li Ni0.8Co0.1Mn0.1O2 at the core to Li Ni0.5Co0.2Mn0.3O2 at the surface within micro-sized spherical particle.This PCG structure stabilizes the particle surface by high Co and Mn content.More importantly,the internal stress is remarkably alleviated by the progressive concentration gradient,effectively boosting the mechanical stability of the micro-size particles upon repeated cycling.Meanwhile,benefited from this PCG structure,the transition metal contents in the bulk are close to Li Ni0.8Co0.1Mn0.1O2,and change mainly on the surface layer to Li Ni0.5Co0.2Mn0.3O2,providing an ingenious approach to maximizing the utilization of high capacity Ni-rich cathode materials with improved structural and surficial stability.Consequently,the progressive concentration gradient cathode material delivers high reversible capacity(189.9 m A h g-1 at 3.0-4.3 V)and cycling stability(86.5% capacity retention after 300 cycles at 1 C).Meanwhile,in order to take full use of the high energy density of this progressive concentration gradient material,by introduction appropriate amount of Al3+ can substantially inhibit the high-voltage phase transition and the cutoff voltage can be extended.As a result,the combination of progressive concentration gradient structure with Al3+ doping provide skillful way to obtain promising high-performance cathode materials with both superior reversible capacity(206.1 m A h g-1 at 3.0-4.5 V)and cycling stability(95.7% capacity retention)to satisfy the growing demands of future electric vehicles. |
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