Atomic layer deposition of Al2O3 on P2-Na0.5Mn0.5Co0.5O2 as interfacial layer for high power sodium-ion batteries

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Abstract

Surface modification is one of the impressive and widely used technique to improve the electrochemical performance of sodium-ion batteries by modifying the electrode-electrolyte interface. Herein, we used the atomic layer deposition (ALD) to modify the surface of P2-Na0.5Mn0.5Co0.5O2 by sub-monolayer Al2O3 coating on the prefabricated electrodes. Phase purity is confirmed using various structural and morphological studies. The pristine electrode delivered an initial discharge capacity of 154 mAh g−1 at 0.5C, and inferior rate performance of 23 mAh g−1 at 40C rate. On the other hand, the interfacial modified cathode with 5 cycles of ALD coating delivers a high capacity of 174 and 45 mAh g−1 at 0.5C and 40C rate, respectively. The Co2+/3+ redox couple is utilized for the faradaic process with high reversibility along with suppressed P2-O2 phase transition. The presence of the Al2O3 layer acts as an artificial cathode electrolyte interface by suppressing the electrolyte oxidation at higher cutoff potentials. This is clearly validated by the reduced charge transfer resistance of surface modified electrodes after cycling at various current rates. Even at an elevated temperature condition (50 °C), interfacial layer significantly improves the safety of the cell and ensures the stability of the cathode.

Introduction

Energy storage sector improves the reliability and utilization of an electrical grid. Renewable and environmentally responsible energy storage solution requires high roundtrip efficiency, long cycle life, low maintenance, and flexible power energy characteristics are good for grid functions. Rechargeable batteries are considered for electrical grid applications. Such batteries find a wide spectrum of applications ranging from portable electronics to large scale energy storage applications [1], [2]. Lithium-ion batteries (LIBs) are first commercialized by SONY in the 1990's by using the layered cathode, LiCoO2 invented by Prof. J.B. Goodenough with graphitic anode [3], [4]. Although, LIB is widely used for portable electronics, but extended as a potential power source towards the automotive industry in the recent past. Therefore, the estimated value of LIB usage is predicted as >95 billion USD in 2025, which leads to the larger consumption of lithium resources and subsequent rise in the price [5]. Utilizing sustainable energy resources is an economically affordable and environmentally safer option for future generations. Compared to lithium, sodium resources are abundant (e.g., the cost of Li2CO3 is 6600 USD/Mt, whereas Na2CO3 is 60 USD/Mt) and widely distributed on the earth surface. Sodium-ion based intercalation compounds are initially studied in the 1970's, but the interest gets subsided due to the rapid progress in the LIB development. Sodium-ion batteries (SIBs) share similar chemical properties with LIB system, and the distribution of sodium is 3–4 orders of magnitude higher than lithium [6], [7], [8], [9]. The only bottleneck in the designing high power SIBs is the higher ionic radius of Na+ (102 Å) compared to Li+ (76 Å) leading to sluggish diffusion kinetics and higher volume change[10]. Graphite anode is the perfect example to understand the difference between the aforesaid two battery systems. The differences in atomic weight and ionic radius have a significant effect and influence on the cyclability and power capability of the battery system. The redox potential of Na+ is slightly higher than Li+. (−2.71 V vs −3.05 V vs. SHE) which reduces the overall cell voltage of SIBs [11], [12]. Aluminium does not form an alloy with sodium; hence, it can be safely used as an anodic current collector compared to expensive Cu [13]. However, industrial production is still pushing to further improve the power capability and lifetime of the battery due to higher demand in various applications. Interfacial engineering is one of the best-pursued approaches in this direction since it influences both the power density and lifeline of the battery. Importantly, this modification certainly prevents inevitable side reactions at electrode and electrolyte at the positive side, which is the main cause for capacity fading. [14], [15], [16], [17].

A wide range of cathode materials was studied as the promising host for SIBs like layered oxides, [14], [18], [18], [19] polyanionic compound [20], [20], [20], [20], [20], organic compounds [21], [21], [22]. Among these, P2 type manganese-based layered oxides are more actively pursued due to its high theoretical capacity, more flexibility in design, and low-cost [23]. The P2 type layered oxides consist of two MO2 layers build by edge-sharing MO6 octahedral units. The Na+ cation resides comfortably in the prismatic sites between these layers which come under one of the broad classifications of Delmas et al. [24], [25]. However, the appearance of multiple voltage plateaus in the charge-discharge process, and Mn3+ dissolution resulting from Jahn-Teller distortion have been a serious concern over the years. Hence, the research activities are keen to replace the Mn3+ with several other metals with either iso or aliovalent substitutions to attain stable structures with better electrochemical properties [26].

Cation doping is a common and simple strategy to improve the rate capability of the LIB [27] and SIBs [28]. In our previous work, we have established that Al-doping can improve the structural stability of layered P2-Na0.5Mn0.5Co0.5O2 [29]. Also, the partial replacement of Mn with Co significantly improve the electrochemical properties of the cathode [30]. Even 10% of Co doping is reported to improve the structural stability of the layered oxides by Bucher et al. [31]. The extraction and re-insertion of Na+ take place smoothly with the disappearance of step-like voltage profiles. The substitution of low valence Co2+ ions could increase the average oxidation state of Mn ions (e.g., Mn4+), thereby enabling the structural stability of the phase by preventing Jahn-Teller dissolution of Mn3+ ions [32]. Secondly, the Ni and Co substitution for the Mn site is said to increase the diffusion coefficient of Na-ions and result in higher power capability [33]. Therefore, Na2/3Mn2/3Co1/3O2 is reported as a high capacity cathode and reversibly able to intercalate 0.5 mol of Na within 1.5 to 4.0 V window [34]. Wang et al. [34] performed a detailed study on the electrochemical performance of the Na2/3MnyCo1-yO2 cathode at higher voltages and predicted that these layered oxides undergo severe structural transition at higher voltages. However, such high voltage (>4.2 V) P2-O2 phase transition induces the capacity fading and poor rate performance. Chen et al. designed an integrated P2/P3 Na0.66Mn0.5Co0.5O2 as a high rate cathode within the limited voltage of 1.5–4.3 V [35]. Wang et al. [36] utilized Co2+/Co3+ redox couple for the first time in sodium layered oxides. Increasing the voltage window could increase the energy density of the layered oxides, but at the sacrifice of material stability subjecting to several factors. The electrode-electrolyte interface is a crucial factor in determining the safety and feasibility of the SIB at elevated temperatures. In common, electrolytes are made of Na+ ion conducting salts dissolved in a mixture of carbonate-based organic solvents, which has low thermal stability with high volatility and flammability that limits the operating temperature and safety of SIBs [37], [38]. Hence interfacial engineering is a must for better stability and safety at elevated temperatures.

Exploring new cathode materials with wide interlayers is one of the ways of designing high power cathode [18], [39], whereas overcoming the setbacks of existing high capacity layered oxides in terms of structure or interfacial properties is another away of tackling the issue [40], [19]. Herein, we adopt surface modification/interfacial engineering as the strategy to design high power layered oxides [41]. Initially, carbon coating is considered as one of the solutions to overcome the poor rate performance [42]. But, the carbon layer reduces the redox capacity of the cathode. In contrast, metal oxide coating certainly improves the rate performance along with yielding higher capacity [43]. Liu et al. [44] reported the Al2O3 coating over Na0.67Ni0.33Mn0.67O2 particulates via the sol-gel method. This solution coating method of Al2O3 encapsulates the surface of the individual particles, thereby increasing the electrical resistance and polarization of the cathode and the capacity is approaching zero at higher rates. Also, the solution coating process failed to provide a homogeneous coating.

Atomic layer deposition (ALD) is one of the successful and simple techniques employed uniform and conformal coating and convincingly validated for several li-ion battery electrodes [16]. It is well known that Al2O3-based coating is non-ideal due to the resistive nature of the oxide. However, for ultrathin layers of Al2O3, lithium can diffuse reasonably well through the layers [15], [16], [45]. The success of this method lies in the (i) very conformal coating on different active materials, (ii) able to control the thickness and (iii) versatility of coating, (iv) suppress CEI layer formation and acts an artificial CEI layer, (v) strengthen the electronic conductivity of the cathode by keeping it together, and (vi) act as a hydrofluoric acid (HF) scavenger [46]. Still, the potential application of this ALD process is at the initial stage of research. Recently, Karthikeyan et al. [43] conducted a conformal coating of Al2O3 by ALD as an artificial CEI layer and found that optimized surface coating could improve the stability and rate capability of the cathode. On the other hand, more in-depth studies are needed to understand the effect of ALD coating on the electrochemical kinetics in SIBs. In this line, we studied the influence of Al2O3 coating thickness, (i.e., the number of cycles) on the prefabricated electrodes of Na0.5Mn0.5Co0.5O2 using ALD. The necessary physical and chemical studies were performed and explained in detail.

Section snippets

Material synthesis

Modified Pechini method (MP) was used for the synthesis of micron-sized P2-type Na0.5Mn0.5Co0.5O2. In a typical process, a stoichiometric amount of sodium acetate (Sigma Aldrich, 99.5%), manganese acetate (Sigma Aldrich, 98%), cobalt acetate (Sigma Aldrich, 98%) were dissolved in distilled water. Then, the citric acid solution was added in a dropwise along with polyethylene glycol as a surfactant under vigorous stirring at 80 °C to encourage the esterification reaction. Ethylene glycol and

Result and discussion

The crystal structure of 50 cycles ALD Al2O3 coated MC900 is analyzed using XRD (Fig. 1a). Compared to other methods like the sol-gel method, the modified pechini (MP) synthesis process is highly facile resulting from the hydroxylation and condensation of the molecules that produce products of required structural and morphological features [47]. Here, the obtained peaks are very sharp which suggests the highly crystalline nature of the synthesized phase. There is no additional impurity peaks

Conclusion

P2-Na0.5Mn0.5Co0.5O2 utilizing Co3+/2+ redox couples were synthesized using polyol synthesis and studied as a high-power cathode for SIBs. We performed interfacial engineering using different cycles of ALD Al2O3 coating and studied its effects on electrochemical kinetics. Surface coating with ALD Al2O3 reduced the average oxidation state of Mn and Co ions. The 5 cycles of ALD modified electrode delivers a high capacity of 174 mAh g−1 and have superior rate capability irrespective of the applied

CRediT authorship contribution statement

Hari Vignesh Ramasamy: Conceptualization, Methodology, Writing - original draft. Pravin N. Didwal: Visualization, Investigation. Soumyadeep Sinha: Formal analysis, Resources. Vanchiappan Aravindan: Formal analysis. Jaeyeong Heo: Formal analysis. Chan-Jin Park: Formal analysis. Yun-Sung Lee: Supervision, Writing - review & editing, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors gratefully acknowledge the financial support from the National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Science, ICT & Future Planning) (No. 2019R1A2C1007620).

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