High electrochemical performance of high-voltage LiNi0.5Mn1.5O4 by decoupling the Ni/Mn disordering from the presence of Mn3+ ions

Abstract

Electrochemical activity in high-voltage spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) is strongly affected by the disordering of Ni/Mn and the presence of Mn 3+ ions. However, understanding the effect of the Ni/Mn disordering or the presence of Mn 3+ ions on electrochemical properties is not trivial because disordering is typically coupled with the presence of Mn 3+ ions. Here, we demonstrate for the first time that the doping of Li instead of Ni increases Ni/Mn disordering, which is decoupled from the presence of Mn 3+ ions. The resultant material has a particle size of~1-2 μm and can achieve 120 mAh g − 1 at 10 C for 50 cycles and further deliver about 60 mAh g − 1 even at a rate of~60 C (1 min discharge). Superior electrochemical performance is achieved by increased solid-solution phase transition behavior, which is caused by increased Ni/Mn disordering during delithiation. By decoupling, we find that the electrochemical properties in LNMO strongly depend on the phase transformation behavior and that the Ni/Mn disordering, rather than Mn 3+ ions, affects the phase transformation by increasing the solid-solution reaction. The fundamental understanding gained from this work could be applied to the development of other phase-separating compounds to improve their electrochemical performance. INTRODUCTIONFor a lithium-ion battery, a high energy density is an essential requirement for novel applications such as plug-in hybrid electric vehicles and electric vehicles. Increasing the redox potential of cathode materials is an effective way to achieve high energy density in the cell. For this purpose, high-voltage spinel LiNi 0.5 Mn 1.5 O 4 is a promising cathode material for LIBs 1-3 because it has a high redox potential of about 4.7 V, which makes its energy density (650 W h kg − 1 ) 20% higher than that of the conventional LiCoO 2 . However, the electrochemical properties of LNMO spinel depend on several factors, such as its structure, 3,4 the quantity of Mn 3+ ions, 5,6 the particle size 2,7 and the morphology of particles. 8 Among these factors, the structure of the spinel has a critical influence on its electrochemical performance. 9-11 The LNMO structure is dictated by the ordering of Ni and Mn at two octahedral sites, which results in two spinel forms: disordered and ordered. The Ni and Mn in the ordered spinel are well ordered in two distinct octahedral sites, 4b sites for Ni and 12d sites for Mn, whereas the Ni and Mn in the disordered spinel are randomly distributed in 16d octahedral sites. 1,12 In the ordered spinel, the ordering of Ni/Mn exists without Mn 3+ ions because a second annealing process at o700°C simultaneously leads to the ordering of Ni/Mn on two distinct octahedral sites and the oxidation of Mn 3+ ions into Mn 4+ ions. However, the disordered spinel shows both the disordering of Ni/Mn and the presence of Mn 3+ ions. The correlation

Controlling Particle Size and Phase Purity of “Single-Crystal” LiNi0.5Mn1.5O4 in Molten-Salt-Assisted Synthesis

“…However, a small degree of Ni/Mn disordering (Fd3̅ m phase) has been shown to enhance cycle performance and rate capability by inducing solid-solution behaviors during (de)lithiation. 42 S1 appears to be the most ordered sample among the three but still contains a small degree of Ni/Mn disordering based on the correlative SEM-Raman studies (Figure 4d) and the first cycle voltage profile (Figure 6a). It indeed exhibited good electrochemical performance by delivering an initial discharge capacity of 130 mA h g −1 , which was higher than S2 (117 mA h g −1 , Figure 6b) and S3 (67 mA h g −1 , Figure 6c).…”

“…To have maximum output energy density, the P4 3 32 phase is preferred. However, a small degree of Ni/Mn disordering ( Fd 3̅ m phase) has been shown to enhance cycle performance and rate capability by inducing solid-solution behaviors during (de)­lithiation . S1 appears to be the most ordered sample among the three but still contains a small degree of Ni/Mn disordering based on the correlative SEM-Raman studies (Figure d) and the first cycle voltage profile (Figure a).…”

Phase Transition Dominated High-Rate Performances of the High Voltage LiNi0.5Mn1.5O4 Cathode: Improvement on Structure Evolution and Ionic Diffusivity by Chromium Doping

“…The variation of lattice parameter upon the evolution of cubic phases is compared through Figure a to c. The discontinuous and steep change in lattice parameter would cause severe strain and stress in the Li x Ni 0.5 Mn 1.5 O 4 , which would add extra barriers impeding the Li ions’ transportation. ,, The extension of the solid-solution phase region would reduce lattice mismatch and alleviate structural stress among different phases, which could benefit both the fast diffusion of Li ions and the structure stability. , …”

“…After electrochemical cycling, the segregations of Cr and Ni ions are similar as before cycling as shown in Figure S3c,d Table 1 compares the region of each cubic phase in the three spinel cathodes upon delithiation, which clearly shows that Cr doping could extend the solid-solution phase region and suppress the nucleation of the third cubic phase in the 1,64,66 The extension of the solid-solution phase region would reduce lattice mismatch and alleviate structural stress among different phases, which could benefit both the fast diffusion of Li ions and the structure stability. 27,33 Although the transition between multiple cubic phases is considered as a reversible process in the spinel cathode, 9,28,30,31,37 With the increase of the potential (vs Li + /Li) scan rate, both the anodic peaks (delithiation) and cathodic peaks (lithiation) gradually become blunt and indistinguishable as shown in Figure 5a, which is a manifestation of the sluggish charge transportation in the LiNi 0.5 Mn 1.5 O 4 . While the redox peaks of Cr doped sample are much sharper as shown in parts b and c of Figure 5.…”

“…Phase transition in the electrode materials plays a crucial role in the chemistry of batteries, which could profoundly influence the energy density and power density. − Many emerging high power applications such as electric vehicles (EVs) and hybrid electric vehicles (HEVs) demand cathode materials with excellent high-rate performances. − The LiNi 0.5 Mn 1.5 O 4 cathode with cubic spinel structure (space group Fd 3̅ m ) is a promising candidate for high power applications owing to its high working potential around 4.7 V vs Li + /Li. − However, subject to the intricate phase transitions upon the lithiation/delithiation reaction, the LiNi 0.5 Mn 1.5 O 4 exhibits unsatisfactory high–rate performances. The formation of the rocksalt-like phase in the delithiation reaction has recently been confirmed by the X-ray diffraction (XRD) and scanning transmission electron microscope (STEM) observation, where some transition-metal (TM) ions migrate into the empty octahedral sites, and then hinders the diffusion of Li ions in the subsequent lithiation reaction. , Furthermore, the evolution of multiple cubic phases during the delithiation/lithiation reaction causes lattice mismatch, which adds extra barriers for Li ions’ transportation in the spinel lattice. ,,− Therefore, eliminating the detrimental phase transitions has been considered as an effective pathway toward excellent high-rate performances for the LiNi 0.5 Mn 1.5 O 4 cathode.…”