Cation order construction in cation-disordered rocksalt cathodes

Lithium-ion batteries have achieved tremendous success in energy storage with a broad range of application. The Li-ion industry is growing to 1TWh of production per year for which approximately one million tons of combined cobalt or nickel will be required. The increasing demand has imposed a large strain on nature resources. Compared to traditional commercialized layered Li(Ni,Mn,Co)O2 (NMC) cathodes, which require expensive cobalt and nickel, the recent development of Li-excess cation-disordered rocksalt (DRX) cathodes widely broaden the possible chemical space from which to draw Li-ion cathodes, including some inexpensive and earth-abundant TMs such as Mn, Ti, Fe, V and Cr. Unlike the well-defined 2-D percolation pathway in layered NMC cathodes, the Li percolation in DRX structures relies on the 3-D quasi-randomly distributed 0-TM channels (no transition metal). Although long-range disordered, the DRX structure still contains short-range cation order (SRO), which has significant impact on Li percolation and electrochemical performance. Therefore, extensive research efforts are required to achieve a better understanding of the correlation between cation order and overall electrochemical performance. In this thesis, three strategies to modify local ordering in DRX cathodes, including metal doping on cation sublattices, synthesis condition control and cation partially order, were carefully evaluated by combining electrochemical tests, advanced characterizations and computational investigations.The first part of the thesis will focus on the cation doping strategy to control the SRO and extractable Li in fluorinated DRX cathodes. Due to the ‘locking effect’ caused by strong bonding between Li and F, not all the Li can be extracted during charging, which compromises the electrochemical performance. A screening based on the bonding strength and ionic radius was performed to identify a suitable dopant to free the locked Li. Mg was chosen because of its large bonding energy with F and similar ionic radius with Li. A detailed theoretical investigation of Li–F SRO using DFT calculations and cluster expansion (CE) Monte Carlo (MC) simulations confirmed that Mg doping results in a greater fraction of extractable Li ions by reducing the frequency of the Li6F configuration. We verified our hypothesis by comparing the electrochemical performance of Li1.25Mn0.45Ti0.3O1.8F0.2 and Li1.25Mg0.1Mn0.45Nb0.2O1.8F0.2. Although both compounds have the same Li-excess and Mn content, the Mg-doped compound delivers higher capacity. Finally, we extended the discussion to a related group of compositions: Li1.333Mn0.667O1.333F0.667 (LMF), Li1.233Mg0.1Mn0.667O1.333F0.667 (ls-LMF), Li1.333Mg0.1Mn0.567O1.333F0.667 (ms-LMF) and Li1.28Mg0.11Mn0.61O1.333F0.667 (ls-LMF) to demonstrate that increased accessible Li, enabled by Mg doping, can also be traded for an improvement of cycle life. The results showed that Mg-doping should be considered in fluorinated DRX cathodes to optimize the performance. The second part of the thesis reports on the solid-state synthesis mechanism of DRX compounds. The synthesis mechanism of a typical DRX oxyfluoride, Li1.2Mn0.55Ti0.25O1.85F0.15 (LMTF), are characterized to investigate the formation of long-range order and short-range order. Experimentally, the formation of long-range disorder followed by short-range order during the synthesis was clearly demonstrated through in-situ transmission electron microscopy, ex-situ X-ray diffraction and pair distribution function analysis. This phenomenon is further explored by performing ab initio calculations for the formation energy of DRX with random structure and with SRO to estimate the reaction kinetics. Based on both experiment and computational results, the SRO feature can be eliminated by simply shortening sintering time and quenching from high temperature. With this strategy, the SRO-free LMTF (35 min) sample exhibited a capacity of >310 mAh g−1 and a specific energy close to 1000 Wh kg−1 and enabled discharge up to 2 A g−1, which is much better than SRO-rich LMTF (4h). The third part of the thesis identifies the cation to anion ratio in synthesis as a key parameter for tuning the structure continuously from a well-ordered spinel, through a partially ordered spinel, to a rocksalt. A series of well selected partially-(dis)ordered spinel cathodes with different degrees of cation over-stoichiometry, Li1.4+xMn1.6O3.7F0.3 (x = 0.07, 0.28, and 0.6), was synthesized via a mechanochemical method. The degree of cation disorder continuously varies with cation to anion ratio, which modifies the voltage profile, rate capability, and charge-compensation mechanism in a rational and predictable way. The results indicate that spinel-type order is most beneficial for achieving high-rate performance as long as the cooperative Li migration from 8a to 16c sites is suppressed. We also found that more rocksalt-like disorder facilitates O redox, which can increase capacity. These findings reveal an important tuning handle for achieving high energy and power in the vast space of partially ordered cathode materials.

Advisor: Ceder, Gerbrand