(4,5,8,9) The WSU/PNNL Na-ion battery which motivated the writing of this Viewpoint utilizes a mixed metal oxide of the composition NaNi 0.68Mn 0.22Co 0.10O 2 with an O3-type layered crystal structure. Layered transition metal dioxides investigated as Na-ion battery cathodes include NaFeO 2, NaNiO 2, NaCrO 2, NaVO 2, and NaTiO 2 and mixed metal dioxides derived from them such as NaFe 1/2Co 1/2O 2, NaNi 1/3Fe 1/3Co 1/3O 2, NaNi 1/3Fe 1/3Mn 1/3O 2, and NaNi 1/4Fe 1/4Co 1/4Mn 1/4O 2. The NaCoO 2 electrode has a rechargeable capacity of about 150 mAh/gram at an average voltage of 3 V. Mixed metal oxide cathodes exhibit additional voltage plateaus in Na-ion cells, reflecting the oxidation states of the different metals being reduced and oxidized during cell discharge and charge. The result is multiple voltage plateaus in the cell’s voltage versus capacity curves reflecting the phase changes ( Figure 2). This conversion between the prismatic and octahedral phases by sliding the MO6 slabs occurs in just about all the layered transition metal oxides when they are used as cathodes in Na-ion batteries. When Na + ions are partly extracted from the O3-phase, those Na + present at prismatic sites become energetically stable and transform the crystal to a P3-phase by the sliding of MO2 slabs without breaking M–O bonds. Sodium ions in the O3-type phase are originally stabilized at edge-shared octahedral sites within the MO6 octahedra. The crystal structure changes of NaCoO 2 in a Na cell begin with the removal of Na during the first charge. Both LiCoO 2 and NaCoO 2 have the same O3 crystal structure consisting of CoO 2 slabs alternately accommodating Li + or Na + ions between the slabs along the c-axis of the A 1– xCoO 2 crystal (where A = Li or Na). They reflect the multiple phase changes of the NaCoO 2 crystals as Na is deintercalated from it to form Na 1– xCoO 2 during charge, and vice versa during discharge. The charge and discharge voltage versus capacity curves of Li/Li 1– xCoO 2 and Na/Na 1– xCoO 2 half-cells compared in Figure 2 (4) reveals stepwise voltage profiles for the Na cell. (1)The NaCoO 2 cathode, like LiCoO 2, is initially brought into the Na-ion cell in the discharged state, and the cell is activated by charging first to form the Na intercalated anode and Na deintercalated cathode in the fully charged cell. The discharged electrodes are on the right-hand side of eq 1. The electrode reactions in a Na-ion battery utilizing hard-carbon (C 6) anode and a layered transition metal oxide, NaMO 2, cathode are depicted in eq 1. In the O3-NaMO 2 phase, Na resides in octahedral sites, while in the P2-phase Na is in prismatic sites. Layered transition metal dioxides, NaMO 2, where M = Fe, Ni, Mn, Co etc., exist in O3 and P2 crystallographic versions. (7) Laboratory test cells and representative prototype cells have been built and evaluated with hard-carbon anodes and cathode materials selected from layered transition metal oxides, transition metal fluorophosphates, and Prussian blue and its analogues. It has a structure similar to that of Li-ion batteries. Figure 1 displays the schematic of a Na-ion battery cell. The chemistry and electrochemistry of electrode materials for Na-ion batteries are sufficiently different from that of their Li-ion counterparts that candidates suitable for practical batteries have become available only recently. It is not a comprehensive review of Na-on batteries as several such reports have appeared elsewhere recently. This Viewpoint, borne out of this enquiry, seeks to answer the question “how comparable are sodium-ion batteries to lithium-ion counterparts“. The excitement encouraged this author to take a deep dive into the original WSU/PNNL reports in ACS Energy Letters, (2,3) examine the state of the art of Na-ion battery technology, and compare it to the mature and ubiquitous lithium (Li)-ion batteries. Naturally this news created a lot of excitement in the battery community and the general public to the extent that some even suggested that a new sodium (Na)-ion battery would replace the expensive lithium-ion batteries. A recent news release from Washington State University (WSU) heralded (1) that “WSU and PNNL (Pacific Northwest National Laboratory) researchers have created a sodium-ion battery that holds as much energy and works as well as some commercial lithium-ion battery chemistries, making for a potentially viable battery technology out of abundant and cheap materials”.
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