Heterosite FePO4的低毒性、高熱穩定性和優異的循環性能，使其成為 Li+、Na+和Mg2+ 等陽離子存儲研究中具有發展潛力的候選材料。Heterosite FePO4的材料製作通常是通過化學氧化脫鋰來得到。在鋰離子的脫鋰過程中，表面化學特性也受到化學試劑的影響。在本研究中，使用過醋酸 (PAA)溶液進行化學脫鋰，自橄欖石結構的磷酸鐵鋰中提取鋰離子，來合成heterosite FePO4。對比於使用傳統的以NO2BF4/AN進行化學脫鋰方法，使用過醋酸溶液製備的heterosite FePO4作為鋰離子和鈉離子的儲存結構，具有更好的充放電容量、循環壽命維持率及高倍率的性能表現。使用 PAA進行化學脫鋰所得到的heterosite FePO4，其優異電化學性能可歸因於原始磷酸鐵鋰表面上碳包覆層具有完整的包覆性及更高的比表面積。
若heterosite FePO4的碳包覆層在化學氧化過程中受到破壞，會導致阻抗上升與容量下降的情形產生。這類型的正極材料在進行充放電的過程中，電解液分解的物質會在正極材料的表面形成CEI層來降低了材料的阻抗，且產生了容量回復的現象。將heterosite FePO4製作成橄欖石結構磷酸鐵鈉為正極的鈉離子電池中進行循環測試，經70次的循環後，其容量保持率高於 85.4 %。使用機械力包覆法將heterosite FePO4或LFP包覆在NCM811正極材料的表面上，可以提升NCM811的倍率性能，NCM811的顆粒表面在包覆了heterosite FePO4或LFP之後，以3C的倍率進行放電時，可以提升容量達到85.7 %及134 %。在鈉金屬電池的應用中，為提高NaFePO4電池體積能量密度，在研究中使用延壓法將鈉金屬與鋁箔結合在一起，使得鈉金屬負極的厚度由200 μm減少至100 μm，在負極減少的100 μm厚度，可用來增加100 μm正極極片塗佈厚度，正極極片的面密度由原來的10 mg/cm2提升至30 mg/cm2，所以增加了2倍NaFePO4電池的體積能量密度。在研究中也展示了鋰鈉雙離子正極材料與混合式充放電模式的方法，證明了鋰離子與鈉離子可以依序嵌入與嵌出同一個結構，並利用不同放電模式可以在鋰鈉雙離子的電池中，分別得到磷酸鐵鋰及磷酸鐵鈉的放電平台及容量，因此拓展了更多鋰離子及鈉離子電池的應用性。
這個研究提供了一種簡單、環保、無需有機溶劑即可合成heterosite FePO4 的方法，並驗證了heterosite FePO4 在鋰電池及鈉電池中應用的可行性。

The low toxicity, high thermal stability, and excellent cycle ability of heterosite FePO4 makes it a promising candidate for the research of cation storage such as Li+, Na+, and Mg2+. Heterosite FePO4 is usually prepared via chemical delithiation process. However, its surface chemistry characteristics are also affected by different chemical agents during the process of lithium ion extraction. In this study, peracetic acid (PAA) solution was used for chemical delithiation, which lithium ions were extracted from an olivine structured lithium iron phosphate to form heterosite FePO4. As a host of lithium and sodium ions, the heterosite FePO4 obtained via chemical delithiation using peracetic acid (PAA) solution performs better charge and discharge capacity, cyclic retention and high rate performance, as compared with the one obtained via traditional chemical delithiation method that use acetonitrile solution with nitronium tetrafluoroborate (NO2BF4 / AN) for chemical delithiation.The superior electrochemical performance of heterosite FePO4 obtained via chemical delithiation using PAA as the chemical agent can be attributed to the better preservation and higher specific surface area of the carbon coating layer on the surface of its pristine lithium iron phosphate after the lithium ion extraction process.
If the carbon coating layer of the heterosite FePO4 is oxidized during the chemical delithiation process, it will lead to an increase in resistance and a decrease in capacity. During charging and discharging processes of such cathode material, the decomposed electrolyte substances will form a CEI layer on the surface of the cathode material that is able to reduce the resistance of the material and result in the phenomenon of capacity recovery. An olivine structure NaFePO4 battery was fabricated using the heterosite FePO4 as the cathode for cyclic testing, and the capacity retention of > 85.4 % after 70 cycles was successfully obtained. Besides, the heterosite FePO4 or LFP were mechanically coated on the surface of NCM811 cathode material and the results showed 85.7 % and 134 % increases in the 3C discharge capacity of NCM811, respectively. In the study of sodium metal battery applications, a cold pressing method was used for the improvement of volumetric energy density of the NaFePO4 battery, in which a piece of sodium metal and an aluminum foil were cold pressed and combined in order to further reduce the thickness of the sodium metal anode from 200 μm to 100 μm. The preserved thickness of 100 μm from the anode side can be applied for the cathode side to further increase cathode electrode thickness by 100 μm which resulted in a two-time increase in the volumetric energy density of the NaFePO4 battery from 10 mg/cm2 to 30 mg/cm2. Lithium and sodium dual-ion cathode material and a hybrid charging and discharging mode were also demonstrated, which proved that lithium ions and sodium ions can be intercalated and de-intercalated into the same structure in sequence. In the lithium and sodium dual-ion battery system, the discharge voltage platforms and capacities of the LiFePO4 and NaFePO4 were obtained using the hybrid discharging mode. As a result of that, we can expect more possible applicability of lithium ion and sodium ion batteries in the future.
This research has provided a simple and eco-friendly method to synthesize heterosite FePO4 without the use of organic solvent and demonstrated its feasibility for of lithium ion and sodium ion batteries applications.

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