Washington State University (WSU) and Pacific Northwest National Laboratory (PNNL) 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.
The team reports one of the best results to date for a sodium-ion battery. It is able to deliver a capacity similar to some lithium-ion batteries and to recharge successfully, keeping more than 80 percent of its charge after 1,000 cycles.
The research, led by Yuehe Lin, professor in WSU’s School of Mechanical and Materials Engineering, and Xiaolin Li, a senior research scientist at PNNL is published in the journal, ACS Energy Letters.
“This is a major development for sodium-ion batteries,” said Dr. Imre Gyuk, director of Energy Storage for the Department of Energy’s Office of Electricity who supported this work at PNNL.
“There is great interest around the potential for replacing Li-ion batteries with Na-ion in many applications.”
Lithium-ion batteries are ubiquitous, used in numerous applications such as cell phones, laptops, and electric vehicles. But they are made from materials, such as cobalt and lithium, that are rare, expensive, and found mostly outside the US.
As demand for electric vehicles and electricity storage rises, these materials will become harder to get and possibly more expensive. Lithium-based batteries would also be problematic in meeting the tremendous growing demand for power grid energy storage.
On the other hand, sodium-ion batteries, made from cheap, abundant, and sustainable sodium from the earth’s oceans or crust, could make a good candidate for large-scale energy storage. Unfortunately, they don’t hold as much energy as lithium batteries.
They also have trouble being recharged as would be required for effective energy storage. A key problem for some of the most promising cathode materials is that a layer of inactive sodium crystals builds up at the surface of the cathode, stopping the flow of sodium ions and, consequently, killing the battery.
“The key challenge is for the battery to have both high energy density and a good cycle life,” said Junhua Song, lead author on the paper and a WSU Ph.D. graduate who is now at Lawrence Berkeley National Laboratory.
As part of the work, the research team created a layered metal oxide cathode and a liquid electrolyte that included extra sodium ions, creating a saltier soup that had a better interaction with their cathode.
Their cathode design and electrolyte system allowed for continued movement of sodium ions, preventing inactive surface crystal build-up and allowing for unimpeded electricity generation.
“Our research revealed the essential correlation between cathode structure evolution and surface interaction with the electrolyte,” Lin said.
“These are the best results ever reported for a sodium-ion battery with a layered cathode, showing that this is a viable technology that can be comparable to lithium-ion batteries.”
The researchers are now working to better understand the important interaction between their electrolyte and the cathode, so they can work with different materials for improved battery design. They also want to design a battery that doesn’t use cobalt, another relatively expensive and rare metal.
“This work paves the way toward practical sodium-ion batteries, and the fundamental insights we gained about the cathode-electrolyte interaction shed light on how we might develop future cobalt-free or low cobalt cathode materials in sodium-ion batteries as well as in other types of battery chemistries,” Song said.
“If we can find viable alternatives to both lithium and cobalt, the sodium-ion battery could truly be competitive with lithium-ion batteries.
“And, that would be a game changer,” he added.
Sodium(Na)-ion batteries (NIBs) have been gaining much attention in the battery field, both academic and industrial, owing to their potential application in large-scale electrical energy storage systems (EESs).(1−3)
Significant efforts have been made in finding suitable electrode materials with desired properties and the determination of structure–property relationships. Because of the large compositional diversity of the structural chemistry, layered oxides are considered as one of the most important electrodes families for NIBs, where the electrochemical performance can be tailored via the introduction of different elements in the hosts.
Na-based layered oxides with the general formula of NaxTMO2 (TM: transition metal) can be categorized into two main structures, O3- and P2-type phases, unlike the layered electrodes of Li-ion batteries (LIBs), which mostly crystallize in the O3-type structure.
O represents that Na ions are accommodated at the octahedral (O) sites and P denotes Na ions at trigonal prismatic (P) sites; the number 2 or 3 represents the number of edge-sharing TMO6 octahedra with the oxygen stacking in ABBA or ABCABC packing (Figure S1), respectively.(4)
It is noteworthy that these layered oxides often experience detrimental phase transitions between O- and P-type structures during the charge–discharge process, making it a challenge to realize good cycling performance.
Compared to the O3-type framework, the P2 structure enables the fast Na+ diffusion owing to the open prismatic diffusion pathways between the TMO2 slabs shown in Figure S1.(5,6) This provides the opportunity to achieve high cycle/rate capabilities.
However, it is established that P2-type electrodes usually deliver a low initial charge capacity of ∼80 mAh g–1 below 4.0 V(7−15) or a low average voltage <3.2 V(11,13,16,17) (Figure S2a). Additionally, they suffer from the detrimental phase transition from P2 to O2 or OP4/′Z′ phases upon charging (desodiation) in Figure 1a, which compromises the cycling stability.(7,9,12,13,16,18)
In order to enhance the properties of P2-type materials, ion-substitution and/or doping, with Li+, Mg2+, Al3+, Ti4+, and Zn2+, having no or fully occupied d orbitals(19−22) and Cu2+ inducing the Jahn–Teller effect,(13) are widely used to alleviate the structural instability.
For example, it has been demonstrated that using 0.05 mol Mg substituting Ni in Na2/3Ni1/3Mn2/3O2 can inhibit the global O2 phase transition to some extent, but instead a local OP4/′Z′ phase transition is induced.(13)
On the other hand, to obtain an increased charge capacity of >100 mAh g–1 below 4.0 V, the TM3+-based P2-type oxides with TMs, such as Mn3+,(12) Fe3+,(9) and Co3+,(23) have been studied.
However, the redox potential of these P2-type cathodes is generally lower than 3.0 V, compromising the overall energy density of the battery, and they often suffer from structural transitions in both the high-voltage (P2 to O2, OP4/′Z′) and low-voltage (P2 to P′2) regions.
Another disadvantage is that these materials are often sensitive to water and moisture in air.(24) From the above, it is clear that it is challenging to realize all demands simultaneously, for which more fundamental understanding is crucial.
A key factor in structural stability of the P2 host is the Na content. Na+ shields the electrostatic repulsions between the TMO2 slabs, in which during the desodiation in the charge process, the decreased shielding of Na, will drive the gliding of the TMO2 slabs, resulting in the structural transition between the P- to O-type stackings.
Therefore, if more Na can be retained in the P2 host upon charging (desodiation), the structural stability can be better maintained during the charge–discharge process. In addition, by analyzing the electrochemical performance of P2 and O3 Na-ion cathodes (Figure S2), we find that O3-type cathodes usually show a higher Na storage capacity than P2-type cathodes in a stable voltage window of 2.0–4.0 V, which is most likely related to the larger initial Na composition (x = 1) of the hosts.
A large amount of Na in P2-type materials can be expected to lower the average oxidation state of the TM ions than that of commonly low Na-content P2 materials, which can raise the 3deg* level of TMs. As a result, the larger number of electrons on the hybrid O(2p)-TM(3d-eg*) orbital will be more accessible at a relatively lower charge voltage, facilitating the charge-transfer reaction.
Therefore, an important goal is to develop P2 materials with high Na content, so that more Na+ to be retained in the NaO2 slabs to prevent the structure transition, while reaching or exceeding the capacity of low Na-content P2 materials (x = 2/3).
To achieve high Na-content P2 materials, we pursue the following rational design strategy. Based on our understanding of Na-ion intercalation chemistry,(25) the ratio between the interlayer distances of d(O–Na–O) and d(O-TM-O) can be used as an indicator to distinguish structural competition between P2- and O3-type layered Na-ion oxides.
The interlayer distance d(O–Na–O) is the average perpendicular distance between the two oxygen sheets enclosing Na ions, and the interlayer distance d(O-TM-O) is the perpendicular distance of two parallel sheets containing transition metal (TM).
This interlayer distance is a result of the interactions of electrostatic cohesion forces and electrostatic repulsive forces between the NaO2 layers and TMO2 slabs. As for the high-Na P2-type oxides, the interlayer distance of d(O–Na–O) will decrease, because the increased Na content will raise the electrostatic cohesion forces between Na+ and O2– layers.
If we assume that the interlayer distance d(O-TM-O) is the determining descriptor, a potential strategy is to substitute TMs in NaxTMO2 with cations having a smaller ion-size and higher oxidation state to reduce its value. Following this strategy, we used P2-type Na2/3Ni1/3Mn2/3O2 as a starting model, which features a large fraction of Mn4+ in the host, having a small ion radius of RCN=6 = 0.53 Å.(26)
It represents a typical low Na content (x = 2/3) electrode, exhibiting a low initial charge capacity of ∼80 mAh g–1 below 4.1 V accompanied by the detrimental P2 to O2 phase transition.(27)
In this work, the Na content in the P2 material was systematically varied from 2/3 to 1 mol per unit by introducing different elements into the pristine structure, such as Li+, Mg2+, Cu2+, Mn3+, Fe3+, and Ti4+, to substitute the Mn4+/Ni2+. Through this approach, several high Na-content materials were obtained with a Na content between 42/54 to 45/54 mol per unit.
— full study —
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.9b13572.
More information: Junhua Song et al, Controlling Surface Phase Transition and Chemical Reactivity of O3-Layered Metal Oxide Cathodes for High-Performance Na-Ion Batteries, ACS Energy Letters (2020). DOI: 10.1021/acsenergylett.0c00700