Dynamic Structure Evolution of Extensively Delithiated High Voltage Spinel Li_1+xNi_0.5Mn_1.5O₄ x < 1.5

Abstract

High voltage spinel is one of the most promising nextgeneration cobalt-free cathode materials for lithium ion battery applications. Besides the typically utilized compositional range of Li x Ni 0.5 Mn 1.5 O4 0 < x < 1 in the voltage window of 4.90−3.00 V, additional 1.5 mol of Li per formula unit can be introduced into the structure, in an extended voltage range to 1.50 V. Theoretically, this leads to significant increase of the specific energy from 690 to 1190 Wh/kg. However, utilization of the extended potential window leads to rapid capacity fading and voltage polarization that lack a comprehensive explanation. In this work, we conducted potentiostatic entropymetry, operando XRD and neutron diffraction on the ordered stoichiometric spinel Li x Ni 0.5 Mn 1.5 O 4 within 0 < x < 2.5 in order to understand the dynamic structure evolution and correlate it with the voltage profile. During the two-phase reaction from cubic (x < 1) to tetragonal (x > 1) phase at ∼2.8 V, we identified the evolution of a second tetragonal phase with x > 2. The structural evaluation during the delithiation indicates the formation of an intermediate phase with cubic symmetry at a lithium content of x = 1.5. Evaluation of neutron diffraction data, with maximum entropy method, of the highly lithiated phase Li x Ni 0.5 Mn 1.5 O 4 with 2 < x < 2.5 strongly suggests that lithium ions are located on octahedral 8a and tetrahedral 4a positions of the distorted tetragonal phase I4 1 amd. Consequently, we were able to provide a conclusive explanation for the additional voltage step at 2.10 V, the sloping voltage profile below 1.80 V, and the additional voltage step at ∼3.80 V.

Asymmetric Lithium Extraction and Insertion in High Voltage Spinel at Fast Rate

“…It has a working voltage of around 4.7 V and a specific capacity of around 135 mAh/g, meaning that the material level energy density is as high as 690 Wh/kg. 3 Benefiting from the three-dimensional ion conduction pathway, this material has good ionic conductivity with a lithium diffusion coefficient typically around 10 −9 cm 2 /s, 4 which is much higher than that of the layered LiNi x Mn y Co 1−x−y O 2 (NMC) materials (10 −11 cm 2 /s), 5 allowing LiNi 0.5 Mn 1.5 O 4 to be charged/discharged at fast rates. 6−8 This material has two kinds of phases, namely ordered and disordered.…”

“…1 Meanwhile, the disordered phase undergoes a combination of both solid-solution reaction and two-phase reaction during charge−discharge. 3 At fast rates, the large overpotential may drive the material to have phase transition behavior distinct from that at slow rates (or close-toequilibrium conditions). In LiFePO 4 , the formation of a nonequilibrium intermediate phase has been observed at a fast rate, and its presence is argued to facilitate high rate capability by decreasing the lattice strain at the phase boundary.…”

“…As o-LNMO is charged to 4.9 V, one Li is extracted from the material as Ni 2+ is oxidized to Ni 3+ and then to Ni 4+ . 3 The reduction of Ni 4+ to Ni 3+ and then to Ni 2+ takes place during discharge. If Mn was redox active, the redox activity of Mn would show as a low voltage plateau in the charge−discharge curves.…”

“…Spinel-type LiNi 0.5 Mn 1.5 O 4 represents an important class of cathode material. , In the context of reducing or even removing expensive and toxic elements such as cobalt from battery materials, LiNi 0.5 Mn 1.5 O 4 has regained a lot of attention as a cobalt-free cathode. It has a working voltage of around 4.7 V and a specific capacity of around 135 mAh/g, meaning that the material level energy density is as high as 690 Wh/kg . Benefiting from the three-dimensional ion conduction pathway, this material has good ionic conductivity with a lithium diffusion coefficient typically around 10 –9 cm 2 /s, which is much higher than that of the layered LiNi x Mn y Co 1– x – y O 2 (NMC) materials (10 –11 cm 2 /s), allowing LiNi 0.5 Mn 1.5 O 4 to be charged/discharged at fast rates. − This material has two kinds of phases, namely ordered and disordered .…”

“…For LiNi 0.5 Mn 1.5 O 4 , it is understood that the ordered phase goes through two two-phase reactions mechanism from Li 1 Ni 0.5 Mn 1.5 O 4 to Li 0.5 Ni 0.5 Mn 1.5 O 4 and then to Li 0 Ni 0.5 Mn 1.5 O 4 during charging and vice versa during discharging . Meanwhile, the disordered phase undergoes a combination of both solid-solution reaction and two-phase reaction during charge–discharge . At fast rates, the large overpotential may drive the material to have phase transition behavior distinct from that at slow rates (or close-to-equilibrium conditions).…”