low carbon - batteries, supercapacitors, and energy sources Q3

Primary vs. secondary batteries

Primary:

• disposable (e.g. zinc-carbon, alkaline)

• suffer from self-discharge (losing 8-20% charge yearly at room temp due to side reactions)

Secondary:

• rechargeable (e.g. lead-acid, NiCd, Li-ion)

• lifespan is limited by internal corrosion, side reactions, and active materials converting into inactive forms over many charge/discharge cycles

Leas-Acid Batteries (standard car batteries): electrochemical reactions

Li-metal vs. Li-ion: Why did Li-Metal fail and the Li-ion solution

Why?

• Early primary batteries used pure Lithium metal because very strong reducing agent with high standard potential (+3.04V).

• Making them rechargeable failed because pure Li forms "dendrites" (spiky crystals) during recharging, pierce the separator, causing short circuits/explosions.

The Li-Ion Solution:

• Instead of pure metal, modern batteries use Chemical Intercalation.

Chemical Intercalation Process

• Intercalation: reversible inclusion/extraction of Lithium ions (Li+) into and out of host lattice without destroying host's crystal structure.

• anode (graphite): Graphite is composed of layered 2D sheets of carbon. During charging, Li+ ions easily slip between these layers (intercalation) and are stored safely as Lix​C6​, preventing explosive dendrite formation.

• cathode: Usually a metal oxide lattice, like Lithium Cobalt Oxide (LiCoO2​).

Li-ion Reactions (during charging)

• note: during discharge these reactions run in reverse

Lithium-Sulfur (Li-S) Batteries

Reactions:

• Anode: 2Li → 2Li+ + 2e-

• Cathode: S + 2Li+ + 2e- → Li2S

• Overall: 2Li + S → Li2S

advantages:

• massive theoretical specific energy (2.6 kWh/kg compared to standard Li-ion at ~0.5 kWh/kg)

• sulfur is incredibly cheap and abundant.

Issues:

• polysulfide shuttle effect

• Solid sulfur has terrible electrical conductivity (10-30 S/cm)

• undergoes massive volume expansion (up to 80%) during discharge, causing severe mechanical stress.

Polysulfide shuttle effect

• During discharge at cathode, sulfur does not instantly become Li2​S.

• S8 → Li2S8 → Li2S6 → Li2S4 → Li2S3→etc. (Discharge @ cathode)

• Li2S → Li2S2 → Li2S3 → Li2S4 → Li2S6 → Li2S8 → S8 (Charge @ cathode)

• Intermediate polysulfides (like Li2​S8​ and Li2​S6​) are highly soluble in organic electrolyte.

• They dissolve, "shuttle" across separator to anode, and ruin battery's capacity.

Sodium-Ion (Na-Ion) Batteries

Advantages:

• Na is 6th most abundant element on Earth - vastly cheaper than Lithium.

• NaO2​ discharge products are more stable than Li2​O2​.

Differences/disadvantages:

• Sodium cation (Na+) is significantly larger than Lithium cation (1.0 Å vs 0.7 Å).

  • because of larger molecular radius, ions move much slower through electrolyte and lattice, resulting in slower charge/discharge rates.

• They have lower specific capacity and lower average voltage (-0.3V compared to Li/Li+).

Mechanism of supercapacitors

• store energy in electrostatic fields

• when charged, accumulation of oppositely charged ionic species gathers at interface between conductive electrode and liquid electrolyte, creating a ‘double layer’

• pros: extremely fast charge/discharge times, 95% efficiency, can endure hundreds of thousands of cycles without degrading

• cons: have low energy density (3-5 Wh/kg) compared to batteries

Why are carbon nanomaterials utilised in supercapacitors?

• capacitance is directly proportional to surface area of electrodes (C=ϵr​ϵ0​A/d).

• They provide incredibly massive specific surface area for ions to accumulate on.

• example to cite:

  • Graphene: Achieves highest energy densities (85.6 Wh/kg) but layers tend to restack.

  • Aligned Carbon Nanotubes (SWCNTs): High power density.

  • Carbon Aerogels: Highly porous, offering up to 1500 m²/g of surface area.

why are supercapacitors used in conjunction with batteries?

• supercapacitors have huge power density but low energy density, they are paired with battery strings (like in hybrid buses/wind turbines).

• supercapacitor handles short, intense power surges (like acceleration or emergency doors), while battery handles steady, long-term energy output.

• this drastically reduces cycling strain on battery and extends its life.

Li-ion chemical intercalation diagram

polysulfide shuttle effect diagram

Ragone chart

Metal-air batteries

use a pure metal anode and extract oxygen directly from the surrounding air for cathode reaction, giving them massive theoretical energy densities

metal-air batteries: Lithium-air batteries

• Reactions (Aprotic Electrolyte):

  • Anode: Li(s)​→Li++e−

  • Cathode: Li++e−+O2​→LiO2∗​ (which then becomes Li2​O2​)

• Advantages:

  • theoretically offer 5-15x more specific energy than standard Li-ion batteries (approaching the energy density of gasoline at ~12 kWh/kg) - massively increase driving range of electric vehicles.

• Issues:

  • Humidity severely degrades cathode and discharge products.

  • pure Lithium anode is extremely reactive and forms dendrites that can pierce the separator

Metal-air batteries: Sodium-Air (Na-Air / Na-Oxygen) Batteries

• Reactions:

  • Na+O2​→NaO2​ (E0 = 2.27 V)

  • 2Na+O2​→Na2​O2​ (E0 = 2.33 V)

  • 2Na+21​O2​→Na2​O (E0 = 1.95 V)

• Comparison to Li-ion / Opportunities:

  • Na is 6th most abundant element on Earth - incredibly cheap and highly scalable for large grid storage compared to Lithium.

  • NaO2​ discharge product is chemically more stable than Li2​O2​ product found in Li-air batteries