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Portable electronics and electric vehicles demand Li-ion batteries (LIBs) with high energy density and electrochemically stable performance. Recent studies in metal-air systems, battery-management modeling, solid-electrolyte/interfacial engineering, and sodium-based dual-ion batteries further show that practical battery performance depends not only on active-electrode chemistry but also on transport, interface, and device-level design considerations [1 -5]. Accordingly, significant effort has focused on developing cathode materials that operate at high potential, deliver high capacity, and maintain high coulombic efficiency. Li-rich layered oxides, typically formulated as Li1.2NixCoyMn0.8 x yO2, have attracted attention because they can provide high discharge capacities (commonly reported at 0.1 -0.5C) and high energy density at ambient temperature [6 -8]. However, these materials still suffer from several well-known limitations, including low initial coulombic efficiency (ICE), poor C-rate capability, and pronounced capacity/voltage decay upon cycling, which are generally linked to structural instability during (de)lithiation and voltage-induced phase evolution [8 -10].

From a crystallographic viewpoint, pristine Li-rich layered oxides are often described as an intergrowth of two components: a monoclinic Li2MnO3-like phase (space group C2/m) and a rhombohedral layered solid-solution phase (space group R3m), frequently represented as LiMO2-type (M = Ni, Co, Mn) (Fig. 1a) [8]. Because these components differ in local ordering, thermodynamic and kinetic behavior, Li + transport and redox processes can depend strongly on the phase fraction

and on cation distributions within the layered framework [10]. Increasing the Ni content in the rhombohedral solid-solution is attractive because Ni redox contributes high capacity and improved electronic conductivity; nevertheless, excessive Ni frequently induces cation disorder (Ni/Li cation mixing), where Ni occupies Li-layer sites and partially blocks Li + diffusion pathways. This disorder is widely associated with reduced ICE, poorer rate performance, and diminished structural stability, and similar effects have been reported for low-Co variants of the Li-rich layered system [11,12].

The monoclinic Li2MnO3-like component is also central to the high capacity of Li-rich layered oxides [8,13]. However, its electrochemical activation introduces additional challenges. During the first charge, oxygen release is commonly observed at high voltages (typically ~4.4 -4.8 V), accompanied by structural rearrangements and partial transformation of the layered framework, which can seed voltage decay and capacity fading upon prolonged cycling [14,15]. More broadly, the charged state promotes dynamic structural evolution in Li-rich layered oxides, including transition-metal (TM) migration from TM layers into Li layers in both rhombohedral and monoclinic domains [14]. Atomistic studies have suggested that TM migration proceeds via intermediate tetrahedral sites and ultimately to neighboring octahedral sites in the Li layer [16]. Once TM ions occupy Li-layer sites, Li + in the same layer can hinder reverse migration, stabilizing a more disordered configuration and progressively impairing Li + transport and voltage retention.

For Mn -Ni Li-rich layered oxides, Ni 2 + is generally considered among the most prone TM species to enter Li-layer sites during synthesis because its ionic radius is close to that of Li + and synthesis conditions can stabilize Ni 2 + [16,17]. Composition tuning can further modulate this tendency. Several studies indicate that increasing Ni content (often coupled with reducing Mn or Co) can shift the average Ni oxidation state toward Ni 3 + to maintain charge neutrality, potentially reducing the driving force for cation disorder and migration in rhombohedral domains [16,17]. In addition, small amounts of Ni in Li-layer sites within the monoclinic component have been proposed to suppress oxygen release during activation and thereby improve electrochemical stability [17]. Therefore, distinguishing and quantifying Ni/Li cation disorder in both rhombohedral and monoclinic components, together with its dependence on phase fraction and Ni/Mn ratio, is essential to understand and improve the electrochemical performance of Co-lean or Co-free Li-rich layered cathodes.

Chemical composition and morphology control provide additional benefits to improve diffusion-limited rate performance. Structurally, α -MnOOH consists of distorted MnO6 polyhedra linked (corner/edgesharing) into chain-like motifs stabilized by hydrogen bonding. α -MnOOH is thermally unstable in air (~200 -500 ◦ C) and can transform through multiple Mn -O intermediates depending on annealing conditions [18]. MnOOH-derived routes have been used to prepare layered and spinel cathodes [19], and a practical advantage is that α -MnOOH template can yield nano-/submicron morphologies that shorten Li + diffusion length in at least one dimension (often on the order of a few hundred nanometers). Because layered structures provide directional Li + diffusion pathways, controlling particle growth and minimizing diffusion length can reduce transport limitations and enhance C-rate capability [20]. In the present series, this previously reported template route is used as a synthesis platform to produce the Co-free Li-rich samples and to compare how the Ni/Mn ratio affects particle-size control and phase/disorder evolution.

Beyond precursor engineering, composition engineering and surface modification strategies, including co-doping, coatings, and alternative synthesis routes; have been extensively investigated to suppress oxygen release and stabilize Li-rich layered cathodes [17,21 -27]. Recent examples include dopant-enabled stabilization (e.g., V-based approaches) and conductive surface modifications aimed at improving ICE, rate capability, and cycling stability [28]. However, a key limitation of many high-performance studies is their reliance on cobalt-containing compositions, which raises concerns related to supply-chain sustainability and

cost.

In this work, Co-free Li-rich layered Li1.2NixMn0.8-xO2 (x = 0.2, 0.3, 0.4) is synthesized via co-precipitation route using α -MnOOH nanorods as a sacrificial Mn template. Although the effect of varying the Ni/Mn ratio has been investigated in related Li-rich Li1.2NixMn0.8-xO2 cathodes, to the best of our knowledge this study is the first to use the previously reported α -MnOOH-template platform to compare a Co-free Li1.2NixMn0.8-xO2 series and directly correlate the Ni/Mn ratio with the R3m/C2/m phase balance, Li/Ni antisite disorder, and rate capability. This combined analysis of composition, phase evolution, and electrochemical behavior allowed us to identify a practical design window at Ni/Mn = 0.6, where a balanced phase fraction and limited cation disorder produce the best overall performance. By systematically tuning the Ni/Mn ratio, we show that electrochemical performance is governed by the coupled effects of (i) the monoclinic/rhombohedral phase balance and (ii) Ni/Li antisite disorder, which together influence Li + transport, initial coulombic efficiency, C-rate capability, and cycling stability. Quantitative XRD Rietveld refinement, combined with electrochemical testing, indicates that performance is maximized at a limited level of Ni occupation in Li-layer sites ( ≈ 2% in our samples), whereas higher Ni/Mn ratios increase cation disorder and shift the phase balance toward monoclinic-dominant compositions, leading to poorer rate capability and stability. This distinction is important because the Rietveld refinement quantifies Ni occupation in the Li layer of the rhombohedral component, whereas the monoclinic-related contribution is tracked through phase fraction, lattice parameters, HRTEM/FFT stacking-fault evidence, and electrochemical activation behavior. The previously reported α -MnOOH-templated route also produces clustered secondary particles assembled from small primary crystallites, shortening diffusion length and helping reveal a practical design window for Co-free Li-rich layered cathodes.

All reagents (analytical/battery grade, Sigma-Aldrich) were used as received. α -MnOOH nanorods were prepared by a hydrothermal route reported previously [13,29]; full conditions are provided in the Supporting Information (SI). For Li -Ni co-precipitation on the α -MnOOH template, the nanorods were dispersed in an ethanol/water mixture (1:1 v/v), followed by addition of lithium acetate (5% Li excess) and nickel acetate to target Li1.2Mn0.8 x Ni x O2 (x = 0.2, 0.3, 0.4). NH4OH was added dropwise to pH 9.4 to promote Ni precipitation on the nanorod surface. The suspension was stirred at 120 ◦ C to obtain a viscous slurry, then vacuum-dried at 140 ◦ C for 72 h to yield a foam-like precursor. The precursors were calcined in air at 800 ◦ C for 12 h (heating rate 7 ◦ C min 1 ) to form the layered oxides. Samples are denoted L-NM 26 (Li1.2Ni0.2Mn0.6O2; Ni/Mn = 0.33), L-NM 35 (Li1.2Ni0.3Mn0.5O2; Ni/Mn = 0.60), and L-NM 44 (Li1.2Ni0.4Mn0.4O2; Ni/Mn = 1.00).

Thermal behavior was evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Chemical characterization was performed using Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy and inductively coupled plasma optical emission spectrometry (ICP -OES). Crystal structure was analyzed by Xray diffraction (XRD). Morphology and elemental distribution were examined by dual-beam focused ion beam/scanning electron microscopy (FIB/SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). Specific surface area was determined by the Brunauer -Emmett -Teller (BET) method. Surface oxidation states (prior to cycling) were examined by X-ray photoelectron spectroscopy (XPS). Instrument models and measurement parameters are provided in the