Rechargeable lithium-ion batteries (LIBs) are critical energy storage devices widely used in smartphones, electric vehicles, and hybrid vehicles due to their high efficiency and versatility. Spinel LiMn₂O₄ stands out as a promising cathode material owing to its eco-friendliness, safety, and cost-effectiveness. However, its practical application is limited by rapid capacity fading during cycling, especially under high current densities or elevated temperatures (55 ℃), caused by Jahn-Teller (J-T) distortion, Mn³⁺ dissolution, and oxygen defects. To address these issues, doping LiMn₂O₄ with elements like Al³⁺, B³⁺, Ni²⁺, or Mg²⁺ is a common strategy to stabilize Mn's average oxidation state (+3.5) and suppress J-T distortion. Concurrently, crystal facet engineering optimizes performance: truncated octahedral particles with dominant (111) facets (low surface energy, dense Mn packing) minimize Mn dissolution, while (100)/(110) facets enhance Li⁺ mobility for improved rate capability. Combining doping and facet control has emerged as a key research focus. For instance, B-doped LiMn_1.96B_0.06O₄ exhibited 110.3 mA·h·g^-1 at 5.0 C with 51.8% capacity retention after 2000 cycles, outperforming undoped LiMn₂O₄. Ni—B co-doping further refined morphology and boosted retention to 70.6%. Similarly, Mg—Al co-doped LiMg_0.03Al_0.10Mn_1.87O₄ retained 76.2% capacity after 1000 cycles at 10.0 C, while Zn—Al doping achieved 70%-75% retention over 2000 cycles. These studies highlight how doping regulates crystal growth, favoring truncated octahedral morphologies with balanced stability and kinetics. Building on this, our work synthesizes LiB_0.08Al_yMn_1.92-yO₄ (y=0.01, 0.02, 0.04, 0.06, 0.08) via solid-state combustion to study Al doping's impact. Results reveal that Al content modulates facet growth: low doping levels yield two-facet (111/100) truncated octahedrons, while higher doping introduces (110) facets, ultimately leading to quasi-spherical particles. This controlled facet producing strategy is designed to optimize structural stability and electrochemical performance for advanced LIBs.

LiB_0.08Mn_1.92O₄ and LiB_0.08Al_yMn_1.92-yO₄ (y=0.01, 0.02, 0.04, 0.06, 0.08) were synthesized by stoichiometric mixing of Li₂CO₃, H₃BO₃, MnCO₃, and Al(OH)(CH₃COO)₂ (total 30 g) with 1.5 g citric acid as a combustion agent. The mixture was ball-milled in ethanol for 10 h, dried, and calcined at 500 ℃ for 1 h. After grinding, it was sintered at 650 ℃ for 6 h. Products were labeled as B_0.08 and B_0.08Al_y. Cathode slurry was prepared by blending active material, carbon black, and PVDF (8∶1∶1) with NMP solvent, followed by 30 min ball milling. The slurry was coated onto Al foil, dried at 80 ℃ for 4 h, and cut into 14 mm discs (active material loading: about 1.0 mg/cm²). CR2032 coin cells were assembled in an Ar-filled glovebox using Li metal anode, LiPF₆ electrolyte (1.0 M in EC/DMC/EMC), and Celgard 2320 separator. Material phases were analyzed via XRD (Cu K_α, 10°-80°). Morphology and microstructure were examined by FE-SEM and TEM. Surface composition and valence states were determined via XPS. Electrochemical tests (Land CT2001A) included cycling (25/55 ℃, 3.0-4.5 V) and rate performance. CV and EIS were conducted using a CHI604D workstation.

XRD analysis confirmed that B_0.08 and B—Al co-doped samples crystallized in the spinel LiMn₂O₄ structure (). B—Al co-doping enhanced crystallinity, with diffraction peaks intensifying until y(Al)≥0.06, beyond which crystallinity declined. A lattice contraction was observed via (400) peak shifts, attributed to smaller B³⁺/Al³⁺(0.0400/0.0535 nm) replacing Mn³⁺(0.0645 nm) and shorter B/Al—O bonds, suppressing J-T distortion. SEM revealed that B—Al co-doping refined particle uniformity and morphology: B_0.08Al_0.02 exhibited truncated octahedral single-crystals with high (111) and minor (100)/(110) facets, while higher Al content (y≥0.04) led to spherical polyhedrons due to reduced facet growth rates. TEM/SAED confirmed improved crystallinity for B_0.08Al_0.02, with distinct lattice fringes (0.476 nm for (111)) and ordered diffraction patterns. XPS verified B³⁺/Al³⁺ incorporation and a higher Mn⁴⁺ ratio (B_0.08Al_0.02∶ Mn³⁺/Mn⁴⁺=47.29%/52.71%), stabilizing the Mn—O framework. Electrochemical tests demonstrated superior performance for B_0.08Al_0.02∶ At 5.0 C, it retained 83.9% capacity after 500 cycles (vs. 71.3% for B_0.08) due to suppressed Mn dissolution and enhanced Li⁺ transport from truncated octahedral morphology. High-rate testing (15.0 C, 20.0 C) showed B_0.08Al_0.02 retained 77.2% capacity after 500 cycles, outperforming B_0.08 (58.2%). Elevated-temperature (55 ℃) cycling revealed B_0.08Al_0.02 maintained 70% capacity retention (200 cycles), attributed to reduced side reactions and structural stability. CV and EIS confirmed faster Li⁺ diffusion (D_Li⁺: 3.14×10^-11 cm²·s^-1) and lower charge-transfer resistance for B_0.08Al_0.02. Kinetic analysis showed a lower activation energy (24.76 kJ/mol vs. 28.53 kJ/mol for B_0.08), indicating an enhanced reaction kinetics.

B—Al co-doped LiB_0.08Al_yMn_1.92-yO₄ (y=0.01, 0.02, 0.04, 0.06, 0.08) cathode materials were synthesized via solid-phase combustion. B—Al co-doping enhanced crystal growth and facet evolution: particles transitioned from truncated octahedrons with two facets (111, 100) to those incorporating (111), (100), and (110) facets as Al content increased. At y(Al)≥0.04, spherical-like polyhedrons dominated. The optimized LiB_0.08Al_0.02Mn_1.90O₄ (B_0.08Al_0.02) exhibited superior crystallinity and well-defined truncated octahedral single-crystals. B_0.08Al_0.02 demonstrated exceptional rate capability and cycling stability. At 1.0 C, 5.0 C, 10.0 C, 15.0 C, 20.0 C, and 55 ℃/5.0 C, initial discharge capacities reached 114.7, 102.8, 86.3, 82.6, 75.3, and 107.4 mA·h·g^-1, respectively. After 500 cycles, capacity retention rates were 83.9% (5.0 C), 81.9% (10 C), 90.3% (15.0 C), 77.2% (20.0 C), and 62.0% (55 ℃/5.0 C), outperforming other samples. CV and EIS analyses demonstrated superior electrochemical properties for the B_0.08Al_0.02, including a higher Li⁺ diffusion coefficient (3.14×10^-11 cm²·s^-1), a lower apparent activation energy (24.76 kJ/mol), and a reduced charge-transfer resistance (114.2 Ω). These results indicate enhanced Li⁺ transport kinetics and improved structural stability.