Titanium dioxide-based materials for alkali metal-ion batteries: Safety and development

Currently, green and high-efficient energy storage technologies are the key to achieving the utilization of renewable resources and the electrification of automobiles [[1], [2], [3]]. Among them, lithium-ion batteries (LIBs), as a secondary cell with high energy density and power density, have shone brightly in many fields. However, the shortage of lithium resources and high prices limit their application in future markets [4]. Recently, other alkali metal-ion batteries [sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs)] are expected to complement or replace LIBs due to their abundant resources, low cost, high energy density, and chemical properties similar to LIBs [[5], [6], [7], [8]]. In order to better understand the differences between them, the composition, physical properties and economy of alkali metal-ion batteries, are provided in Table 1 [[9], [10], [11], [12], [13], [14]]. From the table, it can be seen that alkali metal-ion batteries have significant distinctions in raw material sources and costs. In addition, due to the different properties of charge carriers (Li-ion, Na-ion and K-ion), there are significant differences in the performance of the same materials.

Currently, with the surge in the use of electric vehicles and electronic devices, the issue of battery fires is becoming increasingly prominent, causing concerns among users. In order to enhance the safety characteristics of batteries, researchers have conducted research on electrode materials, separators, and electrolytes, etc. [15] Among them, TiO₂-based materials have important application value in battery safety, but there is still a lack of systematic summary work. In order to help readers quickly access the relevant content, this review focuses on the progress and key issues faced by TiO₂-based materials in preventing battery fires from multiple perspectives.

Due to its abundant resources, non-toxicity, unique physicochemical properties, and easy preparation, TiO₂ has broad application prospects (including environment, energy and biology, etc) [[19], [20], [21], [22]]. For TiO₂, there are eight main polymorphs due to the different stacking patterns of TiO₆ octahedrons in the three-dimensional (3D) space, namely rutile, anatase, brookite, TiO₂-B, TiO₂-R, TiO₂-H, TiO₂-II, and TiO₂-III [[23], [24], [25], [26]]. Among them, the three structures studied extensively are rutile phase, anatase phase and TiO₂-B [25]. For rutile phase TiO₂, it is the most stable form in thermodynamics, while the anatase phase and TiO₂-B are in a metastable phase [23]. In addition, TiO₂ can obtain different morphologies (e.g., nanospheres, nanotubes/wires, nanosheets and bulk) according to different preparation methods. By relying on different polymorphs and morphologies, TiO₂ can exhibit various physicochemical properties, making its application range more extensive [27,28]. In this review, we discussed in detail the role of TiO₂ in electrode materials, separators, and electrolytes.

For alkali metal-ion batteries anode materials, they are usually divided into three categories, including alloy-type (e.g., Sn and Sb), conversion-type (e.g., NiO, SnO₂ and FeO), and intercalation-type (e.g., graphite and TiO₂) anode materials. Among them, for conversion-type and alloy-type anode electrode materials, they usually exhibit significant volume expansion during cycling, which can easily lead to poor cycling life and safety performance [[29], [30], [31]]. Intercalation materials typically undergo minimal volume changes during cycling, and some materials have been commercially applied in LIBs, such as graphite. It is worth noting that carbon materials may experience fire problems during use, mainly due to the unavoidable growth of dendrites during the cycling process (the intercalation potentials for carbon materials of Li⁺, Na⁺ and K⁺ are ∼0.1, ∼0.1 and ∼0.3 V, respectively) [16,32]. As electrode materials, TiO₂ materials are expected to be used to solve the safety issues of batteries. This is mainly because theirs high operating voltage (∼1.7 V vs. Li⁺/Li; ∼0.8 V vs. Na⁺/Na; ∼1.4 V vs. K⁺/K) for alkali-metal ion batteries can be well compatible with traditional organic electrolyte. The SEI formation is significantly suppressed due to the higher operating voltage of TiO₂, though minimal SEI formation may still occur during cycling, which improves the safety performance and cycle performance [23,29,[33], [34], [35]]. It is worth noting that TiO₂ has the lowest intercalation potential in SIBs, which helps improve the energy density of SIBs [36]. Besides, the TiO₂ materials have the following merits: (i) TiO₂ as a substrate enhances the structural stability of other materials due to its small volume expansion during cycling; (ii) the higher capacity can enhance the performances of electrodes; and (iii) the n-type TiO₂ semiconductor can be combined with p-type semiconductors (e.g., FeS₂ and SnS₂) to form heterostructures, effectively adsorbing polysulfides and improving charge transport capacity [8,29]. From the perspective of the charge storage mechanism of TiO₂, it mainly includes faradaic diffusion limited and (pseudo) capacitive charge storage. The former is related to the diffusion limitation inside the bulk, while the latter is related to the surface or near-surface reversible redox reactions [1,37]. By carefully designing, increasing the proportion of capacitive charge storage for TiO₂-based materials during the reaction is more conducive to improving battery rate performance and capacity.

In terms of separators, traditional polyolefin separators for LIBs have a low melting point, which makes it difficult to ensure the safety performance of battery at high temperature [8]. For SIBs and PIBs, the glass separators (GS) commonly used in laboratories does not have high safety characteristics due to large pore sizes and low mechanical properties [38,39]. In addition, the larger thickness for GS reduces the energy density of the battery, and the physical properties of GS are not conducive to practical utilization [39]. Therefore, it is necessary to develop other high safety performance membranes. Recently, researchers have found that membranes containing TiO₂ nanoparticles have good flame retardancy, excellent tensile strength and superior ion conductivity, which have potential application value in solving membrane safety issues and improving battery energy density [8,39]. This may be because TiO₂ nanoparticles can (i) increase the porosity and superhydrophilicity of the membrane, thereby improving the absorption energy and ionic conductivity of the electrolyte; (ii) react with lithium to eliminate dendrites; and (iii) act as a blockage to inhibit dendrites from penetrating the membrane [[40], [41], [42]].

In order to address the leakage risk of traditional liquid electrolyte, researchers are vigorously developing SSE [43,44]. This is mainly due to its advantages such as non-flammability, low reactivity with electrode material, and the ability to act as a separator to improve battery energy density. Currently, SSE has been extensively studied in LIBs and SIBs, including oxide-based solid electrolytes [e.g., Li_3.3La_0.56TiO₃ (LLTO), LiTi₂(PO₄)₃ (NASICON) (lithium superionic conductor), Li₁₄Zn(GeO₄)₄ (LISICON) (sodium superionic conductor), and Na₃Zr₂Si₂PO₁₂ (NZSP)], sulfide-based solid electrolytes (e.g., Li₂S-P₂S₅ and Li₂S-P₂S₅-MS_x), polymers [e.g., polyoxyethylene, poly (ethylene oxide) (PEO), poly (methyl methacrylate) (PMMA) and polyvinylidene fluoride (PVDF)], and lithium salts (e.g., LiClO₄, LiN(CF₃SO₂)₂ (LiTFSI), LiAsF₆ and LiPF₆) [45]. Among them, polymers with higher crystallinity usually have lower ionic conductivity. For oxide-based SSE, the key issue it faces is how to suppress the growth of lithium dendrites. Recent studies have found that TiO₂ has potential application value in SSE, especially in polymer and oxide-based SSE [44,46]. In summary, the addition of TiO₂ can (i) serve as a stabilizer, improving the toughness of SSE and disrupting the electrical continuity of dendrites; (ii) as a coating material to fill the surface of grain boundaries and coated particles, suppressing the formation of dendrites, increasing the critical current density of the battery, and improving the wetting of the SSE; and (iii) as a modifier, improving the electrochemical stability, thermal stability, and electrochemical performance of the battery [43,44].

From the above analysis, it can be seen that TiO₂ plays an important role in preparing refractory materials and increasing the initial temperature of thermal runaway due to its unique properties (see Table 2). It can be foreseen that with the gradual deepening of research, TiO₂ may play a greater role in suppressing battery fires.

At present, the practical application of TiO₂-based materials in batteries still faces many challenges, including the challenge of TiO₂ itself and the challenge of TiO₂ in different modules. From the perspective of TiO₂ itself, in recent years, with the increasing demand for titanium resources and the gradual decline of high-grade titanium ore (such as ilmenite and rutile) reserves, TiO₂ raw material sources are gradually limited [47]. In addition, due to the rising price and uneven distribution of raw materials, as well as the high cost of large-scale preparation of powder TiO₂, the price of TiO₂-based materials is difficult to further reduce. The above factors also affect the large-scale application of TiO₂-based materials in alkali-metal ion batteries in the future [48]. In terms of application, due to the different usage scenarios for TiO₂-based materials, different optimization strategies need to be adopted. Table 3 summaries the main challenges faced by TiO₂-based materials in different modules [1,19,44,[49], [50], [51]]. For example, for alkali metal-ion batteries anode materials, the high charge/discharge plateau of TiO₂ usually affects the output power of the battery; as an n-type semiconductor, the electronic conductivity and ionic conductivity for TiO₂ are relatively poor, which affects the rate performance of the battery. As an effective component of separator materials, the hydrophilicity, ionic conductivity and thermal stability for TiO₂-based materials need to be optimized. The interface resistance between TiO₂ and the SSE host material also needs to be further solved.

Overall, for alkali metal-ion batteries, research on TiO₂-based materials mainly focuses on basic research, and the corresponding research is mainly in the laboratory stage. Therefore, it is urgent to explore the performances of TiO₂-based materials and investigate their behavior in the market. At present, the optimization of TiO₂-based materials by researchers mainly focuses on two aspects: the structural regulation of material, and the improvement of the performance for composite material. This paper analyzes the important achievements of TiO₂-based materials in synthetic preparation, mechanism analysis and performance optimization in detail, clarifies the key technical problems facing these materials, and points out the next research direction of TiO₂-based materials. Although numerous studies have explored TiO₂-based materials for alkali-ion batteries from various perspectives, research on the safety aspects of these batteries remains notably limited. To date, there is a lack of systematic integration of safety considerations within the broader context of TiO₂-based alkali-ion battery research. While such integration would significantly enhance the coherence and organization of this review, the current state of research does not permit a comprehensive treatment of this critical aspect. Given the constraints, this review is structured around three primary categories based on the limited safety-related studies conducted over the past five years: lithium-ion batteries (Section 2), sodium-ion batteries (Section 3), and potassium-ion batteries (Section 4). By organizing the discussion in this manner, we aim to provide a clearer framework for understanding the safety challenges and advancements in TiO₂-based alkali-metal ion batteries. It is our hope that this review will stimulate further interest and investigation into the safety of these materials, as addressing safety concerns is a crucial step toward the successful commercialization of TiO₂-based alkali-metal ion batteries.