Ga‑Doped LiNi0.5−xGa_xMn1.5O4 Spinel Cathodes: Enhanced High‑Temperature Stability and Rate Capability

Abstract

Using a sol‑gel route, LiNi_0.5−xGa_xMn_1.5O₄ (x = 0, 0.04, 0.06, 0.08, 0.10) cathodes were synthesized and systematically studied. X‑ray diffraction confirms a disordered Fd3m spinel phase; gallium doping suppresses the Li_xNi_1−xO secondary phase and increases cation disorder, which improves electronic conductivity and Li⁺ diffusion. Electrochemical tests at 25 °C and 55 °C show that the optimally doped LiNi_0.44Ga_0.06Mn_1.5O₄ retains 98.3 % of its 115.7 mAh g⁻¹ capacity at 3 C after 100 cycles and delivers 98.4 % capacity retention at 1 C/55 °C after 50 cycles. The Ga‑doped material also exhibits superior rate performance, outperforming the pristine LNMO at high discharge rates.

Background

LiNi_0.5Mn_1.5O₄ (LNMO) is a leading high‑voltage cathode (4.7 V vs Li/Li⁺) with a theoretical energy density of 658 Wh kg⁻¹. Its three‑dimensional Li⁺ diffusion network and high operating potential make it attractive for electric‑vehicle and grid storage. However, LNMO suffers from two major issues: (1) the formation of Li_xNi_1−xO impurities during high‑temperature calcination, which block Li⁺ pathways, and (2) electrolyte decomposition at the high working voltage, leading to capacity fade and poor high‑temperature stability.

Previous strategies to mitigate these problems include transition‑metal doping (Cr, Mg, Y, Ce, Al, Cu, Ga) and surface coatings (Al₂O₃, BiFeO₃). Gallium, in particular, has been reported to suppress Jahn–Teller distortions and stabilize the spinel lattice, but systematic studies on its effect across a range of concentrations and at elevated temperatures are lacking.

Results and Discussion

Structural and Morphological Analysis

XRD patterns (Fig. 1) show all samples crystallize in the disordered Fd3m space group (JCPDS No. 80‑2162). Only the undoped LNMO exhibits minor Li_xNi_1−xO peaks; Ga doping completely eliminates this secondary phase, confirming successful substitution into the lattice. The I₃₁₁/I₄₀₀ ratio peaks at 0.9216 for x = 0.06, indicating optimal structural stability.

FT‑IR spectra (Fig. 2) reveal the strongest 621 cm⁻¹ (Mn–O) band, characteristic of the Fd3m structure. The I₅₈₈/I₆₂₁ ratio reaches a minimum (0.708) for x = 0.06, signifying maximal cation disorder and enhanced ionic conductivity.

SEM images (Fig. 3) show uniform octahedral grains (~200 nm) for all compositions; EDS confirms Ga incorporation proportional to the nominal x value.

Electrochemical Performance

Rate capability (Fig. 5a) demonstrates that Ga‑doped samples maintain high capacity at 3 C. The optimally doped LiNi_0.44Ga_0.06Mn_1.5O₄ delivers 115.7 mAh g⁻¹ at 3 C versus 87.3 mAh g⁻¹ for pristine LNMO. This improvement is attributed to reduced impurity phase, higher electronic conductivity (lower R_ct), and increased Li⁺ diffusion coefficient (D_Li⁺).

Cycling stability (Fig. 6a,b) shows the 0.06 Ga sample retains 98.4 % of its initial capacity after 50 cycles at 1 C/55 °C, while pristine LNMO drops to 74 %. At 3 C, 0.06 Ga retains 98.3 % after 100 cycles, outperforming the undoped material (80 %).

dQ/dV analysis (Fig. 7) indicates reduced polarization for the Ga‑doped cathode, with a minimal Ni³⁺/Ni⁴⁺ peak separation of 0.011 V compared to 0.037 V for pristine LNMO, confirming better Li⁺ reversibility.

EIS measurements (Fig. 8a) reveal the lowest charge‑transfer resistance (86.7 Ω) for x = 0.06, matching the highest D_Li⁺ (7.99 × 10⁻¹¹ cm²s⁻¹). The trend of decreasing R_ct with increasing Ga up to 0.06, followed by a rise, mirrors the structural disorder and conductivity data.

Conclusions

Ga doping via a sol‑gel route effectively suppresses the Li_xNi_1−xO impurity, increases cation disorder, and forms a protective surface layer. The LiNi_0.44Ga_0.06Mn_1.5O₄ cathode delivers >98 % capacity retention at 1 C/55 °C and superior high‑rate performance, offering a viable pathway for high‑temperature Li‑ion batteries.

Methods

Material Synthesis

LiNi_0.5−xGa_xMn_1.5O₄ powders were prepared by dissolving stoichiometric amounts of Li(CH₃COO)·2H₂O, Mn(CH₃COO)·4H₂O, Ni(CH₃COO)·4H₂O, and Ga(NO₃)₃·xH₂O in deionized water. Citric acid was added, and the mixture was heated to 80 °C under stirring. The pH was adjusted to 7 with NH₄OH, yielding a gel that was dried at 110 °C. The dried precursor was calcined at 650 °C for 5 h, ground, and then sintered at 850 °C for 16 h. The resulting powders (Ga‑0 to Ga‑0.1) were characterized as described below.

Characterization

XRD (Cu Kα, 36 kV, 20 mA) collected 10–80° at 4°/min. FT‑IR (Nicolet 6700), SEM (JEOL 6700F), and EDS provided morphological and compositional data.

Electrochemical Testing

Electrodes were prepared by mixing 90 wt% active material, 5 wt% Super P, and 5 wt% PVDF in NMP, cast onto Al foil, dried, and pressed into 14 mm disks. CR2032 coin cells with Li foil counter/reference were assembled in Ar (≤0.1 ppm H₂O/O₂). Electrolyte: 1 M LiPF₆ in EC:PC:EMC (1:2:7 v/v/v). Galvanostatic charge–discharge (3.5–4.95 V) was performed at 25 °C and 55 °C using a LAND system. EIS (0.01 Hz–100 kHz, 5 mV) was conducted on a CHI600A workstation.

Abbreviations

LiNi_0.5−xGa_xMn_1.5O₄

Ga‑doped LiNi_0.5Mn_1.5O₄ spinel cathode

LNMO

LiNi_0.5Mn_1.5O₄

R_ct

Charge‑transfer resistance

D_Li⁺

Li⁺ diffusion coefficient

SEI

Solid electrolyte interphase

Etc.

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